Aims. The liner design is a key determinant of the constraint of a reverse total shoulder arthroplasty (rTSA). The aim of this study was to compare the degree of constraint of rTSA liners between different implant systems. Methods. An implant company’s independent 3D shoulder arthroplasty planning software (mediCAD 3D shoulder v. 7.0, module v. 2.1.84.173.43) was used to determine the jump height of standard and constrained liners of different sizes (radius of curvature) of all available companies. The obtained parameters were used to calculate the stability ratio (degree of constraint) and angle of coverage (degree of glenosphere coverage by liner) of the different systems. Measurements were independently performed by two raters, and intraclass correlation coefficients were calculated to perform a reliability analysis. Additionally, measurements were compared with parameters provided by the companies themselves, when available, to ensure validity of the software-derived measurements. Results. There were variations in jump height between rTSA systems at a given size, resulting in large differences in stability ratio between systems. Standard liners exhibited a stability ratio range from 126% to 214% (mean 158% (SD 23%)) and constrained liners a range from 151% to 479% (mean 245% (SD 76%)). The angle of coverage showed a range from 103° to 130° (mean 115° (SD 7°)) for standard and a range from 113° to 156° (mean 133° (SD 11°)) for constrained liners. Four arthroplasty systems kept the stability ratio of standard liners constant (within 5%) across different sizes, while one system showed slight inconsistencies (within 10%), and ten arthroplasty systems showed large inconsistencies (range 11% to 28%). The stability ratio of constrained liners was consistent across different sizes in two arthroplasty systems and inconsistent in seven systems (range 18% to 106%). Conclusion. Large differences in jump height and resulting degree of constraint of rTSA liners were observed between different implant systems, and in many cases even within the same implant systems. While the immediate clinical effect remains unclear, in theory the degree of constraint of the liner plays an important role for the dislocation and notching risk of a rTSA system. Cite this article: Bone Jt Open 2024;5(10):818–824

Aims. Mid-level constraint designs for total knee arthroplasty (TKA) are intended to reduce coronal plane laxity. Our aims were to compare kinematics and ligament forces of the Zimmer Biomet Persona posterior-stabilized (PS) and mid-level designs in the coronal, sagittal, and axial planes under loads simulating clinical exams of the knee in a cadaver model. Methods. We performed TKA on eight cadaveric knees and loaded them using a robotic manipulator. We tested both PS and mid-level designs under loads simulating clinical exams via applied varus and valgus moments, internal-external (IE) rotation moments, and anteroposterior forces at 0°, 30°, and 90° of flexion. We measured the resulting tibiofemoral angulations and translations. We also quantified the forces carried by the medial and lateral collateral ligaments (MCL/LCL) via serial sectioning of these structures and use of the principle of superposition. Results. Mid-level inserts reduced varus angulations compared to PS inserts by a median of 0.4°, 0.9°, and 1.5° at 0°, 30°, and 90° of flexion, respectively, and reduced valgus angulations by a median of 0.3°, 1.0°, and 1.2° (p ≤ 0.027 for all comparisons). Mid-level inserts reduced net IE rotations by a median of 5.6°, 14.7°, and 17.5° at 0°, 30°, and 90°, respectively (p = 0.012). Mid-level inserts reduced anterior tibial translation only at 90° of flexion by a median of 3.0 millimetres (p = 0.036). With an applied varus moment, the mid-level insert decreased LCL force compared to the PS insert at all three flexion angles that were tested (p ≤ 0.036). In contrast, with a valgus moment the mid-level insert did not reduce MCL force. With an applied internal rotation moment, the mid-level insert decreased LCL force at 30° and 90° by a median of 25.7 N and 31.7 N, respectively (p = 0.017 and p = 0.012). With an external rotation moment, the mid-level insert decreased MCL force at 30° and 90° by a median of 45.7 N and 20.0 N, respectively (p ≤ 0.017 for all comparisons). With an applied anterior load, MCL and LCL forces showed no differences between the two inserts at 30° and 90° of flexion. Conclusion. The mid-level insert used in this study decreased coronal and axial plane laxities compared to the PS insert, but its stabilizing benefit in the sagittal plane was limited. Both mid-level and PS inserts depended on the MCL to resist anterior loads during a simulated clinical exam of anterior laxity. Cite this article: Bone Jt Open 2023;4(6):432–441

Constrained implants with intra-medullary fixation are expedient for complex TKA. Constraint is associated with loosening, but can correction of deformity mitigate risk of loosening?. Primary TKA's with a non-linked constrained prosthesis from 2010-2018 were identified. Indications were ligamentous instability or intra-medullary fixation to bypass stress risers. All included fully cemented 30mm stem extensions on tibia and femur. If soft tissue stability was achieved, a posterior stabilized (PS) tibial insert was selected. Pre and post TKA full length radiographs showed. i. hip-knee-ankle angles (HKAA). ii. Kennedy Zone (KZ) where hip to ankle vector crosses knee joint. 77 TKA's in 68 patients, average age 69.3 years (41-89.5) with OA (65%) post-trauma (24.5%) and inflammatory arthropathy (10.5%). Pre-op radiographs (62 knees) showed varus in 37.0%. (HKAA: 4. o. -29. o. ), valgus in 59.6% (HKAA range 8. o. -41. o. ) and 2 knees in neutral. 13 cases deceased within 2 years were excluded. Six with 2 year follow up pending have not been revised. Mean follow-up is 6.1 yrs (2.4-11.9yrs). Long post-op radiographs showed 34 (57.6%) in central KZ (HKKA 180. o. +/- 2. o. ). . Thirteen (22.0%) were in mechanical varus (HKAA 3. o. -15. o. ) and 12 (20.3%) in mechanical valgus: HKAA (171. o. -178. o. ). Three failed with infection; 2 after ORIF and one with BMI>50. The greatest post op varus suffered peri-prosthetic fracture. There was no aseptic loosening or instability. Only full-length radiographs accurately measure alignment and very few similar studies exist. No cases failed by loosening or instability, but PPF followed persistent malalignment. Infection complicated prior ORIF and elevated BMI. This does not endorse indiscriminate use of mechanically constrained knee prostheses. Lower demand patients with complex arthropathy, especially severe deformity, benefit from fully cemented, non-linked constrained prostheses, with intra-medullary fixation. Hinges are not necessarily indicated, and rotational constraint does not lead to loosening

Introduction. The interaction between the mobile components of total elbow replacements (TER) provides additional constraint to the elbow motion. Semi-constrained TER depend on a mechanical linkage to avoid dislocation and have greater constraint than unconstrained TER that rely primarily in soft tissue for joint stability. Greater constraint increases the load transfer to the implant interfaces and the stresses in the polyethylene components. Both of these phenomena are detrimental to the longevity of TER, as they may result in implant loosening and increased damage to the polyethylene components, respectively[1]. The objective of this work was to compare the constraint profile in varus-valgus and internal-external rotation and the polyethylene stresses under loads from a common daily activity between two semi-constrained TER, Coonrad/Morrey (Zimmer-Biomet) and Discovery® (DJO), and an unconstrained TER, TEMA (LimaCorporate). Methods. We developed finite element (FE) models of the three TER mechanisms. To reduce computational cost, we did not include the humeral and ulnar stems. Materials were linear-elastic for the metallic components (E. Ti6Al4V. =114.3 GPa, E. CoCr. =210 GPa, v=0.33) and linear elastic-plastic for the polyethylene components (E=618 MPa, v=0.46; S. Y. =22 MPa; S. U. =230.6 MPa; ε. U. =1.5 mm/mm). The models were meshed with linear tetrahedral elements of sizes 0.4–0.6 mm. We assumed a friction coefficient of 0.02 between metal and polyethylene. In all simulations, the ulnar component was fixed and the humeral component loaded. We computed the constraint profiles in full extension by simulating each mechanism from 8° varus to 8° valgus and from 8° internal to 8° external rotation. All other degrees-of-freedom except for flexion extension were unconstrained. Then, we identified the instant during feeding that generated the highest moments at the elbow[2], and we applied the joint forces and moments to each TER to evaluate the stresses in the polyethylene. To validate the FE results, we experimentally evaluated the constraint of the design with highest polyethylene stresses in pure internal-external rotation and compared the results against those from a FE model that reproduced the experimental setup (Fig.1-a). Results. For each design, the constraint profiles in varus-valgus (Fig.2-a) were similar to internal-external rotation (Fig.2-b). All designs showed a lax zone in which the mechanisms rotated freely and an engagement zone in which the mobile components contacted, resulting in load transfer. The laxity of the Coonrad/Morrey and the Discovery® was similar and lower than that of the TEMA. After engagement, the stiffness of the TEMA was less than that of the Discovery® and the Coonrad/Morrey. The TEMA showed the lowest polyethylene stresses of all three designs under demanding loads during feeding. Only Discovery® and Coonrad/Morrey had zones reaching permanent deformation (Fig.3). For the Coonrad/Morrey, with the highest polyethylene stresses, the experimental and computational constraint profiles were similar (Fig.1-b). Discussion. The TEMA unconstrained design transferred less moment than semi-constrained designs, reducing the burden on the implant interfaces. Moreover, the TEMA design had lower stresses in the polyethylene components due to the combination of less constraint and a lack of sharp edges on the articular surfaces. For any figures or tables, please contact the authors directly

Introduction. Studies have shown that increased implant conformity in total knee arthroplasty (TKA) has been linked to increased constraint and thus rotational torque at the bone/implant interface. Anterior stabilized (AS) tibial inserts were designed to compensate for excessive AP motion in less-conforming cruciate-retaining (CR) tibial inserts. However, increased constraint may affect implant loading. Therefore, the purpose of this study is to model rotational prosthesis constraint based on implant-specific data and to compare rotational torque and 3D contact forces in implants with CR-lipped and AS tibial inserts during normal gait. Methods. A previously reported knee joint contact model was updated to include rotational torque due to prosthesis constraint (ASTM F1223(14)). Piecewise multiple linear regression with manually selected cutoff points was used to determine estimates of AP force, ML force, and rotation torque as functions of AP displacement, ML displacement, knee external rotation, respectively, and knee flexion angle from standard data. These functions were used to estimate total moment contribution of the prosthesis from measured knee displacement/rotation angles. Estimates were incorporated into the contact model equilibrium equations as needed by the model. As the model parametrically varies muscle activation coefficients to solve for the range of physiologically possible forces at each time point, the reported force/torque values are the mean across all solutions at each time point. Rotational torque and three dimensional contact forces were calculated for 14 informed-consented subjects, five with AS tibial inserts (1/4 m/f, 67±10 years, 29.2±4.4 BMI, 1/4 right/left) and nine with CR-lipped TKRs (2/7 m/f, 64±6 years, 30.6±5.8 BMI, 4/5 right/left). Rotational torque waveforms were compared using statistical nonparametric mapping; 3D contact forces were compared at mean timing of the flexion/extension moment peaks using independent samples t-tests. Results. Waveform analysis of rotational torque found no significant differences between implant types. CR- lipped inserts showed an initial peak internal rotation torque during weight acceptance and continued with a pattern of internal rotation throughout stance. Peak torque for AS inserts also occurred during weight acceptance, but it varied between internal/external rotation torque. Additionally, after weight acceptance, AS subjects showed a pattern of external rotation torque. Mean axial force, medial-lateral shear force, and anterior-posterior shear force waveforms were similarly shaped between implant groups. Flexion and extension moment peaks occurred at 23% and 74% stance on average. There were no significant differences in three-dimensional knee joint contact forces between groups at either time point. Discussion. There were different patterns of rotational torque between groups. Implants with lipped CR inserts tended to undergo internal rotation torques that peaked during weight acceptance. Torque seen in implants with AS inserts was also largest during weight acceptance, but greatly varied between internal and external rotation, before settling in a pattern of external rotation for the remainder of stance. This may be due to constraints added by AS insert geometry. In conclusion, a model of rotational torque due to implant constraints has been developed; increased implant constraint increased the external rotation torque experienced by the implant and may also affect shear forces at the implant surface. For any figures or tables, please contact authors directly

Background. The constraint of total knee replacement (TKR) implants is not simply defined and many of the factors that influence it are not well understood. Variability in the constraint of different TKR implants designed for the same indication (e.g. cruciate-retaining, or posterior-stabilized) have been previously demonstrated, but these differences among implants have yet to be simply quantified. Furthermore, the relative importance of several variables on the implant constraint remains unknown. The purpose of this study was to quantify the differences in constraint that exist between different implant designs, and to examine the effects of axial load and flexion angle on the constraint of current cruciate-retaining (CR) TKR components. Methods. Four contemporary CR TKR designs underwent laxity testing using a multi-axis mechanical test machine. Implants were tested at flexion angles of 0°, 20°, 90° and maximum flexion and axial loads of 712 N (1 BW) and 1424 N (2 BW). Friction-free motion in all secondary degrees of freedom was allowed. Force-displacement curves were generated for each testing condition in both anterior-posterior (AP) and rotational tests. AP constraint (N/mm) and rotational constraint (Nm/deg) were then calculated. Results. Differences in the AP and rotational constraint of the various implants were identified. The rotating platform implant was the most AP constrained and least rotationally constrained of the implants studied. Among the fixed bearing implants, NexGen CR had less AP and rotational constraint than the Genesis II and PFC Sigma. Doubling the applied axial load during the laxity tests resulted in a significant increase in both AP (p<0.0001) and rotational (p<0.0001) constraint. AP and rotational constraint decreased with increasing flexion angles. Conclusion. The constraints of four contemporary CR TKR designs were quantitatively compared and ranked according to their calculated constraints. Differences in the constraint of these designs were identified. This quantitative analysis is an important step towards the creation of a clinically meaningful constraint index that would aid surgeons in choosing the optimal implants for their TKR patients. Additionally, the analysis of the effects of axial load and flexion angle on constraint has implications which may influence how in vitro testing of implants is conducted as well as clinical implications in terms of how the constraint envelope of a given implant may influence the feel and function that a TKR patient experiences

Introduction. Pre-clinical assessment of total knee replacements (TKR) can provide useful information about the constraint provided by an implant, and therefore help the surgeon decide the most appropriate configurations. For example, increasing the posterior tibial slope is believed to delay impingement in deep flexion and thus increase the maximal flexion angle of the knee, however it is unclear what effect this has on anterior-posterior (AP) constraint. The current ASTM standard (F1223) for determining constraint gives little guidance on important factors such as medial- lateral (M:L) loading distribution, flexion angle or coupled secondary motions. Therefore, the aim of the study was to assess the sensitivity of the ASTM standard to these variations, and investigate how increasing the posterior tibial slope affects TKR constraint. Methods. Using a six degree of freedom testing rig, a cruciate-retaining TKR (Legion; Smith & Nephew) was tested for AP translational constraint. In both anterior and posterior directions, the tibial component was displaced until a ‘dislocation limit’ was reached (fig. 1), the point at which the force-displacement graph started to plateau (fig. 2). Compressive joint loads from 710 to 2000 N, and a range of medial-lateral (M:L) load distributions, from 70:30% to 30:70% M:L, were applied at different flexion angles with secondary motions unconstrained. The posterior slope of the tibial component was varied at 0°, 3°, 6° and 9°. Results. AP translation was significantly larger at 60° and 90° flexion (22 ± 1 mm and 24 ± 1 mm respectively) than at 0° (14 ± 1 mm), whilst increasing the compressive joint load increased the force required to translate the tibia to limits of AP constraint at all flexion angles tested. When the M:L load distribution was shifted medially, a coupled internal rotation was observed with anterior translation and external rotation with posterior translation; this was reversed with a lateral shift in load distribution. It was also found that increasing the posterior slope of the tibial tray moved the neutral position of the tibia relative to the femur more anteriorly at all flexion angles tested. The constraint under anterior drawer was then reduced with increasing slope, which meant that the tray dislocated at lower drawer force and translations. Conclusions. When intraoperative tibial bone cuts are made, surgeons should be aware that by increasing posterior slope angles the TKR may offer less anterior constraint under body-weight loads, therefore relying more heavily on surrounding soft-tissue and muscle action to prevent dislocation. The ASTM test protocol could be refined to stipulate the variation of the M:L loading distribution. It has been shown to vary between patients and activities, and the AP constraint and associated secondary motions in this study were very sensitive to this distribution. The secondary motions observed should be measured and recorded to provide more information about the device's stability characteristics. The tests could also be extended to include a higher axial load such as 2000 N, approximately three times body weight, in order to investigate coupled rotations and M:L distribution effects whilst under normal walking gait loads

Summary Statement. The constraint behavior of total knee arthroplasty (TKA) prosthesis usually has to be physically tested. This study presents a computer simulation model using finite element analysis (FEA) and demonstrates its effectiveness in predicting the femorotibial constraint behavior of TKA implants. Introduction. TKA prostheses are semi-constrained artificial joints. A well-functioning TKA prosthesis should be designed with a good balance between stability and mobility, meaning the femorotibial constraint of the artificial joint cannot be excessive or too lax. To assess the constraint behavior of a TKA prosthesis, physical testing is usually required, and an industrial test standard has been developed for this purpose. Benefiting from technological advancement, computer simulation has become increasingly useful in many industries, including medical device research and development. FEA has been extensively used in stress analysis and structural evaluation of various orthopaedic implants. This study presented an FEA-based simulation to evaluate the femorotibial constraint behavior of TKA prosthesis, and demonstrated the effectiveness of the method by validating it through physical testing. Methods. A Cruciate Retaining (CR) TKA prosthesis design (Optetrak Logic CR, size 3, Exactech, FL, USA) was used in this study. The prosthesis system consists of a femoral component, a tibial insert, and a tibial baseplate. CAD models of the implants assembled at 0° of flexion were used for the simulation. Finite element models were generated using 10-node tetrahedral elements, with all materials considered linear elastic. Boundary conditions were set up according to the ASTM F1223 standard. The tibial baseplate was fixed distally. A constant compressive force (710 N) was applied on the femoral component. Nonlinear Surface-Surface-Contact was defined at the femorotibial articulating surfaces as well as between the tibial insert and tibial baseplate. A coefficient of friction of 0.2 determined from the physical test was input into the simulation. The femoral component was driven under a displacement-controlled scheme to slide along the anterior-posterior (AP) direction on the tibial insert. At each time step, constraint force occurring at the articulating surface was derived from the reaction force at the distal fixation of the tibial baseplate. The force-displacement curve was plotted by combining the results of all time steps to characterize the constraint behavior of the prosthesis. A nonlinear FEA solver (NX Nastran SOL601, Siemens, TX, USA) was used to solve the simulation. In addition, five samples of the prostheses were physically tested per ASTM F1223. Simulation results were compared to the physical testing. Results. The simulation successfully captured the movement of contact location and pressure along the movement of the femoral component. The force-displacement curve predicted by the simulation exhibited a very close hysteresis loop profile as the results of physical testing. Using the curve slope from 0 to 5 mm to characterise the constraint in the most relevant displacement range, the simulation predicted 45.7 N/mm anteriorly and 36.4 N/mm posteriorly, which are less than 10% different from the physical testing results (46.4 N/mm anteriorly and 39.6 N/mm posteriorly). Discussion/Conclusion. This study demonstrated that the simulation was able to closely predict the femorotibial constraint behavior of the TKA prosthesis under ASTM F1223 testing. The simulation results resembled the physical test results not only in the general profile of the curve but also in the magnitude of slope values. The increased difference at the far anterior region could be related to the fact that no material nonlinearity was considered in the current simulation, a factor that could be improved in future studies. A validated simulation method could be very useful in TKA prosthesis design. Since no physical prototypes are required, design evaluation and optimization can be achieved in a much easier and faster manner

Introduction. Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. Femorotibial constraint is a key property of a TKA prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually used for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses may be used. A CRC tibial insert usually has a more conforming sagittal profile especially in the anterior aspect to provide increased constraint to prevent paradoxical femoral translation during knee flexion. A quantitative understanding of the constraint behavior of a prosthesis design is critical to ensure its functional outcome. Using a validated computer simulation, this study evaluated the anterior-posterior (AP) constraint of two types of tibial inserts (CR and CRC) from a same TKA product family. Methods. Both the CR and CRC prostheses are from the same TKA product family (Optetrak Logic, Exactech, USA). Three sizes (sizes 1, 3, and 5) from each product line were included in this study. Computer simulations using finite element analysis (FEA) were performed at 0° flexion per ASTM F1223 standard [1] (Figure 1). The simulation has been validated with physical testing (more details submitted in a separate abstract to ISTA 2013). Briefly, FEA models were created with all materials considered linear elastic. The tibial baseplate was distally fixed and a constant compressive force (710 N) was applied to the femoral component. Nonlinear Surface-Surface-Contact was established at the articulating surfaces. A coefficient of friction of 0.1 was assumed for all articulations [2]. The femoral component was driven under a displacement-controlled scheme to slide along AP direction on the tibial insert. Constraint force occurring at the articulation was derived from the reaction force at the distal fixation. A nonlinear FEA solver was used to solve the simulations. Results. The force-displacement curves predicted by the simulation exhibited the hysteresis loop appearance for both CR and CRC inserts (Figure 2). The anterior aspect of the CRC curves showed a steeper raise than the CR curves, and the trend was consistent across sizes. Taking the slope from 0 to 5 mm range, the anterior constraint of the CRC insert was significantly greater than the CR insert, while the posterior constraint of the CRC insert was also slightly greater (Figure 3). Discussion/Conclusion. The increased AP constraint of the CRC insert revealed in the study is consistent with the design geometry and functional intent of the device. With a much increased anterior lip, the CRC insert is expected to provide substantially greater anterior constraint than the CR insert to prevent paradoxical femoral translation for patients without a fully functioning PCL. The CRC insert is also expected to provide slightly increased posterior constraint due to its gently elevated posterior lip. This study quantitatively demonstrated the effect of design geometry on the outcome constraint function of different TKA prostheses

Introduction. Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. A well-functioning TKA prosthesis should be designed with a good balance between stability and mobility, meaning the femorotibial constraint of the artificial joint should be appropriate for the device's function. To assess the constraint behavior of a TKA prosthesis, physical testing is typically required, and an industrial testing standard has been developed for this purpose [1]. Computer simulation has become increasingly useful in many industries, including medical device research and development where finite element analysis (FEA) has been extensively used in stress analysis and structural evaluation. This study presents an FEA-based simulation to evaluate the femorotibial constraint behavior of TKA prosthesis, and demonstrated the effectiveness of the method by validating through physical testing. Methods. A Cruciate Retaining (CR) TKA prosthesis design (Optetrak Logic CR, Exactech, USA) was used in this study. CAD models of the implants assembled at 0° of flexion were used for the simulation. Finite element models were generated using with all materials assumed linear elastic. Boundary conditions were set up according to the ASTM F1223 standard (Figure 1). The tibial baseplate was fixed distally. A constant compressive force (710 N) was applied on the femoral component. Nonlinear Surface-Surface-Contact was defined at the femorotibial articulating surfaces. Coefficient of friction was determined from physical test. The femoral component was driven under a displacement-controlled scheme to slide along the anterior-posterior (AP) direction on the tibial insert. At each time step, constraint force occurring at the articulating surface was derived from the reaction force at the distal fixation of the tibial baseplate. A nonlinear FEA solver (NX Nastran SOL601, Siemens, USA) was used to solve the simulation. In addition, five samples of the prostheses were physically tested, and the results were compared with the simulation. Results. The simulation successfully captured the movement of contact location and pressure along the movement of the femoral component (Figure 2). The force-displacement curve predicted by the simulation exhibited a very close hysteresis loop profile as the results of physical testing (Figure 3). Using the curve slope from 0 to 5 mm to characterize the linear constraint, the simulation predicted 45.7 N/mm anteriorly and 36.4 N/mm posteriorly, which are less than 10% different from the physical testing results (46.4 N/mm anteriorly and 39.6 N/mm posteriorly). Discussion/Conclusion. This study demonstrated that the simulation was able to closely predict the femorotibial constraint behavior of the TKA prosthesis under ASTM F1223 testing. The simulation results resembled the physical testing results not only in the general curve profile but also in the magnitude of slope values. The increased difference at the far anterior region could be related to the fact that no material nonlinearity was currently considered, which could be improved in future studies. A validated simulation method could be very useful in TKA prosthesis design. Since no physical prototypes are required, design evaluation and optimization can be achieved in a much easier and faster manner

Summary Statement. Femorotibial constraint is a key property of a total knee arthroplasty (TKA) prosthesis and should reflect the intended function of the device. With a validated simulation methodology, this study evaluated the constraint of two TKA prostheses designed for different intentions. Introduction. TKA prostheses are semi-constrained artificial joints. Femorotibial constraint level is a major property of a prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually indicated for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses with additional built-in constraint may be indicated. A CRC prosthesis usually consists of a CR femoral component and a tibial insert which has a more conforming sagittal profile to offer an increased femorotibial constraint. This study evaluated the anterior-posterior (AP) constraint behavior of two lines of prostheses (CR and CRC) from a same TKA product family. Using a validated computer simulation approach, multiple sizes of each product line were evaluated. Methods. Both the CR and CRC prostheses are from the same TKA product family (Optetrak Logic, Exactech, FL, USA) and share identical femoral components and tibial baseplates. The CRC tibial inserts have a more conforming sagittal profile than the CR tibial inserts, especially in the anterior aspect. Three sizes (sizes 1, 3, and 5) from each product line were included in this study. Computer simulations using finite element analysis (FEA) were performed to evaluate the femorotibial constraint of each prosthesis per ASTM F1223 standard [1]. The simulation has been validated by comparison with physical testing (more details submitted in a separate paper to CORS 2013). Briefly, FEA models were created using 10-node tetrahedral elements with all materials considered linear elastic. The tibial baseplate was distally fixed and a constant compressive force (710 N) was applied to the femoral component. Nonlinear Surface-Surface-Contact was established at the articulating surfaces, as well as between the tibial insert and the tibial baseplate. A coefficient of friction of 0.1 was assumed for all articulations [2]. The femoral component was driven under a displacement-controlled scheme to slide along AP direction on the tibial insert. Constraint force occurring at the articulation was derived from the reaction force at the distal fixation; thus, the force-displacement curve can be plotted to characterise the constraint behavior of the prosthesis. A nonlinear FEA solver (NX Nastran SOL601, Siemens, TX, USA) was used to solve the simulations. Results. The force-displacement curves predicted by the simulation exhibited the hysteresis loop appearance for both CR and CRC prostheses. The profile of the curves was generally consistent across different sizes for both product lines. The anterior constraint of the CRC prosthesis was significantly greater than the CR prosthesis. The posterior constraint of the CRC prosthesis was also slightly greater. Larger sizes exhibited reduced constraint compared to smaller sizes. Discussion/Conclusion. The increased constraint of the CRC prosthesis revealed in the study is consistent with the geometrical characteristics and the functional intent of the device. The CRC tibial insert is expected to provide significantly greater anterior constraint than the CR prosthesis to prevent paradoxical femoral translation when the patient's PCL is not fully functioning. The CRC tibial insert is also expected to provide slightly increased posterior constraint due to its elevated posterior lip. The observed hysteresis loop appearance is consistent with physical testing and the existence of friction. The reduced constraint on larger sizes is functionally desirable to offer proportional translation freedom. This study demonstrated the effectiveness of the simulation approach in quantifying the constraint behavior of different TKA prosthesis designs

Purpose. Although classic teaching holds that the least amount of constraint should be implanted, there is very little in the literature to substantiate this. This study attempts to quantify the influence of constraint and various indications upon functional outcome following aseptic first time revision knee arthroplasty. The null hypothesis was that the level of constraint and indication for surgery would not influence the functional outcome following revision knee replacement. Methods. A single centre prospective study was performed to examine the outcome for 175 consecutive total revision knee replacements performed between 2003 and 2008 with a minimum follow-up of two years. Patient reported outcome data was used to determine the influence of final level of component constraint and its relationship with primary indication for surgery. Results. All patients were found to have a significant improvement for WOMAC pain, function and stiffness score (p< 0.001) and physical functioning, role physical, bodily pain and social functioning components of SF-36 score (p< 0.05). 69% were satisfied with the overall procedure. WOMAC function, pain and stiffness score was significantly worse for patients revised for instability (27%) compared to that for aseptic loosening (46%). A significantly higher proportion of patients were satisfied with the procedure (p< 0.001), had a better quality of life (p=0.004) and would have the surgery (p=0.005) again in the aseptic loosening group as compared to the instability group. Revision to a higher level of constraint did not improve knee function irrespective of the primary indication for surgery. Conclusion. Revision for an instability pattern irrespective of choice of new device was met with significantly poorer functional outcome

The goals of total knee arthroplasty (TKA) are to relieve pain, restore function, and provide a stable joint. In regard to types of implants, the workhorses are posterior cruciate retaining (CR), posterior stabilised (PS), and posterior stabilised constrained (PSC) designs. However, the continuum of constraint now ranges from standard cruciate retaining (CR-S) to CR lipped (CR-L), to anterior stabilised (CR-AS), to posterior stabilised, to a PS “plus” that fits with a PS femoral component but provides a small degree of varus-valgus constraint, to a PSC or constrained condylar type of device, to a rotating hinge. As the degree of deformity, bone loss, contracture, ligamentous instability and osteopenia increases, so does the demand for prosthetic constraint. When deformity is minimal and the posterior cruciate ligament (PCL) is intact and functional, a CR-S device is appropriate. For moderate deformity with deficiency or compromise of the PCL, a CR-AS or posterior stabilised device is warranted. In severe cases, with attenuation or absence of either of the collateral ligaments, a constrained condylar device, with options of stems, wedges and augments, is advisable. In salvage situations, when both collaterals are compromised, a rotating hinge should be utilised. Prerequisites for use of a CR-S device are an intact PCL, balanced medial and lateral collateral ligaments, and equal flexion and extension gaps. With a CR-L bearing, a slight posterior lip is incorporated into the sagittal profile of the component to provide a small amount of extra stability in the articulation. It is important for the surgeon to be aware of the design features of the implant system he or she is using. For example, in a system where the CR-S bearing has 3 degrees of posterior slope and the CR-L bearing has no slope, the thickness of a CR-L bearing posteriorly is approximately 2mm greater than the CR-S. A CR-L bearing is indicated for to provide stability where the flexion gap is just slightly looser than the extension gap and the PCL is intact. If the patient's knee is somewhat lax in flexion and stable in extension, a CR-L bearing may help to stabilise both the flexion and extension gaps yet still allow the knee to obtain full extension, whereas if a CR-S bearing in the next thicker size is used to stabilise the flexion gap, a flexion contracture may result. CR-AS bearings are required less frequently. They are indicated when the flexion and extension gaps are balanced, but the PCL is deficient, and the surgeon does not want to change to a PS design, which requires additional bony resection of intercondylar notch. The PCL is one of the strongest ligaments in the knee, and affords inherent stability to the TKA. In flexion, the PCL not only affords AP stability, but also imparts flexion gap stability, acting as a lateral stabiliser of the medial compartment and a medial stabiliser of the lateral compartment. The PCL has a crucial role with respect to femoral rollback, which imparts added efficiency to the extensor mechanism. PCL retention is a more biologically preserving operative intervention than PS-TKA

Introduction. Revision for instability has supplanted revision for aseptic loosening and revision for osteolysis since the advent of improved polyethylene inserts with changes in both sterilization techniques and cross-linking. Having the ability to judiciously choose a higher level of constraint may be beneficial in complex primary total knee arthroplasty (TKA) scenarios which can not be balanced through traditional surgical methods. The purpose of this work was to investigate short term outcomes and survivorship in cases where a greater stabilizing insert was used with a posterior stabalizing (PS) femur to address instability in flexion or extension. Methods. Two high volume TKA centers retrospectively reviewed cases in which a greater stabilizer insert was used with a primary PS knee system. The studied insert had +/− 2 degrees of varus-valgus coronal restraint as opposed the standard with no coronal constraint. The study inserts had 7 degrees of transverse plane rotational freedom. The inserts were used when extension balance was not achieved despite the usual soft tissue releases and a thicker insert resulted in a flexion contracture statically during the procedure. This situation typically occurred in the following patient groups: valgus knees with medial collateral (MCL) stretching, iatrogenic MCL injury, varus knees with lateral ligament complex stretching, the “double-varus” knee, and patients with a previous high tibial osteotomy. Intra-operatively patients were taken through a range of motion and trial implants were then placed. A cruciate retaining trial insert was then used to assess stability so that a true assessment could be made of ligament balance. Bone cuts were checked before ligament release. The usual releases were then performed to achieve balance including subperiosteal releases medially and laterally and pie-crusting when indicated. Repeat trial reductions were then performed once the final implants were cemented in place again using the cruciate retaining insert. If the soft tissue releases did not achieve balance and a thicker insert resulted in a flexion contracture then the greater stabilizer insert was selected over the PS insert. Knee Society Score and plain radiographs were collected at pre-op, 2 year and 5 year follow-up. Results. One hundred seventy two cases with 2 year minimum follow-up and 41 cases with 5 year minimum follow up were assessed. All patients had good to excellent Knee Society Scores with good range of motion and pain relief. There were no aseptic revisions of the TKA's over this period. Specifically, there were no revisions for loosening, osteolysis, instability, or post breakage. Conclusions. A more stabilized tibial post insert which provides valgus-varus constraint but permits rotational freedom may provide needed stability in select primary situations without predisposing to early post failure or implant loosening or lysis

Introduction. We wished to quantify the influence of constraint and various principal indications upon functional outcome following aseptic first time revision knee arthroplasty. Methods. In this single centre, prospective study we have looked into the outcome of 175 total revision knee replacement performed between 2003 and 2008 at a minimum follow-up of 1 year. Data was refined to allow for examination exclusively of those cases where the level of constraint was increased from cruciate retaining or cruciate sacrificing knee replacement to either non linked constrained implant (condylar constrained) or linked constrained (rotating hinge) prosthesis. Results. At one year significant improvement (p<0.001) of WOMAC pain, function and pain score was seen. Significant improvement (p<0.05) was noted in physical functioning, role physical, bodily pain and social functioning components of SF-36 score. 69% were satisfied with the overall procedure. WOMAC function, pain and stiffness score was significantly worse for patients revised for instability (27%) as compared to that for aseptic loosening (46%). A significantly higher proportion of patients were satisfied with the procedure (p<0.001); had a better quality of life (p=0.004) and would have the surgery (p=0.005) again in the aseptic loosening group as compared to the instability group. Level of constraint did not make any statistically significant difference in the outcome within either group. The postoperative outcome scores for patients receiving two different levels of constraint was statistically not different. We found that increasing the level of constraint to linked constrained implant significantly (p<0.001) deteriorated the ability to descend from stairs (Figure 4). Conversely it had a positive influence (p=0.01) on ability to walk on flat. Conclusion. Patients undergoing revision surgery for instability exhibited poorer functional knee scores at 1 year. The degree of constraint within the revision construct did not significantly alter this change irrespective of aetiology

Introduction. Modern implant systems offer a variety of options to address the posterior cruciate ligament (PCL) and afford stability in primary total knee arthroplasty (TKA). One system has three bearing options for cruciate retaining (CR) TKA: standard CR bearing (CR-S) with 3° posterior slope and no posterior lip; CR lipped bearing (CR-L) with no slope and small posterior lip, and deep-dished anterior stabilized bearing (CR-AS) with large anterior build-up to compensates for deficient or lax PCL. We previously reported on 1940 patients (2449 CR-TKA) at early follow-up comparing these three CR bearings and found CR-AS bearings had greater improvement in ROM and lower rate of manipulation despite their use in patients with greater preoperative deformity and lower ROM. The purpose of this study was to review a broader series by a single surgeon-designer with minimum 2-year follow-up to determine if there were differences between bearing types in terms of clinical outcomes and survival. Methods. From 2003 to 2014, 3348 patients (4500 knees) underwent primary CR-TKA and were available for review with minimum 2-year follow-up. An algorithmic approach to constraint was employed, choosing the least amount necessary to afford stability. Bearings used were 3883 CR-S (86%), 256 CR-L (6%), and 361 CR-AS (8%). CR-S bearings were used in patients with less preoperative tibiofemoral deformity (p=0.0316), less flexion contracture (p<0.0001), and better ROM (p<0.0001), but more mediolateral instability (p<0.0001) than CR-AS bearings. Results. Mean follow-up overall was 5.7 years (range, 2–14). Overall 106 TKA (2.4%) have been revised (46 fully, 56 partial, 4 unknown). Indications for revision were instability in 40 (all CR-S), 28 infection, 10 aseptic loosening, 8 arthrofibrosis, 7 periprosthetic fracture, 3 extensor related, 2 heterotopic ossification, 3 pain, and 5 other or unknown. Chi-squared analysis showed no difference between bearing groups for overall revision (p=0.1724). However, revision for instability was greater for CR-S (1.0%) versus CR-AS bearings (p=0.0454, Fisher's exact). Frequency of manipulation was no different between bearing groups (p=0.8512). CR-AS patients had greater improvement than CR-S patients in ROM (4.4° versus 1.6°, p=0.0003), and Knee Society pain (39.3 versus 35.9, p=0.0002), clinical (53.5 versus 47.0, p<0.0001), and function (18.9 versus 16.0, p=16.0). CR-L patients had greater improvement than CR-S in pain (38.2, p=0.0367) and clinical score (53.6, p<0.0001). Discussion. In this large single surgeon series, Kaplan-Meier survival free of any revision was similar between bearing groups and excellent overall with 96.0% (±0.49%) at 14.1 years. No revisions for instability were observed with CR-L or CR-AS bearings, but were the most frequent cause of revision with CR-S bearings. Manipulation rates were similar between bearing groups. ROM, pain, clinical, and functional improvements were greater with a deep-dished, anterior-stabilized bearing

The Acrobot®, an active constraint “hands-on” robotic system, gives navigation cues to the surgeon, and also assists him in the surgery, using active software constraints if he tries to depart from the preoperative plan. It has just entered clinical trials. We report the first 5 cases. The Acrobot® system for precision total knee arthroplasty comprises the following components:. 1. A CT-based planning system. 2. The limb positioning system. 3. The Acrobot’s hardware components:. a gross positioning device with separate brakes and encoders, locked off for safety during the procedure,. a fully back-driveable low force robot, and. a force control handle on the robot close to the high-speed milling tool. 4. The Acrobot’s software which:. imports the preoperative plan,. allows anatomic registration. provides navigation,. physically assists the surgeon perform his plan. Each patient’s knee scores were monitored and postoperative CT scan was compared with the preoperative plan. Seven robot assisted arthroplasties have been performed. No significant complications have been encountered. The Knee and Womac Scores show that the procedure is safe and comparable to conventional surgery in the early postoperative period. The envelope of error on postoperative CT scans has been within the accuracy of the method of measurement, at < 1 mm and < 10 without the outliers which haunt every clinical series. The Acrobot® system for total knee arthroplasty has completed its preliminary trial satisfactorily. It provides a handson operation but with robotic levels of accuracy. It is suitable for conventional open surgery, but its real place will be in the arena of minimally invasive unicondylar knee arthroplasty, hip arthroplasty and resurfacing, and in the spine, where active constraint will prevent potentially dangerous surgical errors

Total knee arthroplasty (TKA) is a major orthopaedic intervention. The length of a patient's stay has been progressively reduced with the introduction of enhanced recovery protocols: day-case surgery has become the ultimate challenge. This narrative review shows the potential limitations of day-case TKA. These constraints may be social, linked to patient’s comorbidities, or due to surgery-related adverse events (e.g. pain, post-operative nausea and vomiting, etc.). . Using patient stratification, tailored surgical techniques and multimodal opioid-sparing analgesia, day-case TKA might be achievable in a limited group of patients. The younger, male patient without comorbidities and with an excellent social network around him might be a candidate. Demographic changes, effective recovery programmes and less invasive surgical techniques such as unicondylar knee arthroplasty, may increase the size of the group of potential day-case patients. . The cost reduction achieved by day-case TKA needs to be balanced against any increase in morbidity and mortality and the cost of advanced follow-up at a distance with new technology. These factors need to be evaluated before adopting this ultimate ‘fast-track’ approach. . Cite this article: Bone Joint J 2015;97-B(10 Suppl A):40–4

INTRODUCTION. The specific factors affecting wear of the ultrahigh molecular weight polyethylene (UHMWPE) tibial component of total knee replacements (TKR) are poorly understood. One recent study demonstrated that lower conforming inserts produced less wear in knee simulators. The purpose of this study is to investigate the effect of insert conformity and design on articular surface wear of postmortem retrieved UHMWPE tibial inserts. METHODS. Nineteen NexGen cruciate-retaining (NexGen CR) and twenty-five NexGen posterior-stabilized (NexGen PS) (Zimmer) UHWMPE tibial inserts were retrieved at postmortem from fifteen and eighteen patients respectively. Articular surfaces were scanned at 100×100μm using a coordinate measuring machine (SmartScope, OGP Inc.). Autonomous mathematical reconstruction of the original surface was used to calculate volume loss and linear penetration maps of the medial and lateral plateaus. Wear rates for the medial, lateral and total articular surface were calculated as the slope of the linear regression line of volume loss against implantation time. Volume loss due to creep was estimated as the regression intercept. Student t-tests were used to check for significant. RESULTS. The NexGen CR and NexGen PS patient groups were approximately the same age at time of implantation (mean±SD: 72.1±9.9 and 68.7±8.8 years respectively, p=0.260) and implantation times were not significantly different (8.7±3.1 and 9.1±3.7 years, p=0.670). Both groups showed high variability in wear scars. No significant difference in wear rates on the total surface (mean±SE: 11.89±5.01 mm. 3. /year vs. 11.09±4.18 mm. 3. /year, p=0.905). However, NexGen CR components showed significantly higher volume loss due to creep than NexGen PS components (70.22±47.07 mm. 3. vs. 31.30±41.15 mm. 3. , p=0.007). These results were reflected on the medial and lateral sides, with no significant differences in wear rates on the medial side (p=0.856) or lateral side (p=0.633) and higher volume losses due to creep associated with the NexGen CR components. While NexGen CR and NexGen PS showed a near equal mean percentage of volume loss on the medial side (CR: 52.4±11.7%, PS: 52.5±11.6%), a paired t-test showed that NexGen PS components showed a higher volume loss on the medial side (p=0.056), NexGen CR components did not (p=0.404). DISCUSSION. The combination of higher conformity and more kinematic constraint in NexGen CR components may create larger contact areas with higher stresses, leading to higher volume loss due to creep observed in this study. However, these factors did not produce increased wear rates in the population. Constrained components may maintain more loading on medial side and limit sliding distance on lateral side, causing more wear medially. Total wear rates were very similar and resembled the previously reported rate of 12.9 ± 5.97 mm. 3. /year for retrieved Miller-Galante II (Zimmer) components, which features a near flat articulating surface. These findings indicate that materials factors may be most important in producing wear and that higher conformity alone does not decrease wear. For any figures or tables, please contact authors directly

Introduction: Active Robots have been shown to be effective at performing arthroplasty, but some hesitation has been felt by the surgical world. The lack of human interface in the procedure has been one of the stumbling blocks towards wider acceptance. The Acrobot has been developed, at Imperial College London, in collaboration with University College London to allow the surgeon to perform the surgery himself, but with active constraint, preventing him from taking too much bone, or straying into soft tissue. Materials and methods: A preoperative planning system is used, based on ct data acquired without fiducial markers. Semi-automated segmentation is performed. The surgeon then performs the virtual surgery on the bones on screen, allowing precise sizing, and orientation. The safe field of activity is then defined, within which the surgeon is free. The patient is positioned on the operating table and immobilised. Anatomic registration is then performed, and when sufficient accuracy obtained, the milling procedure is begun. A high speed electric milling tool is used, and with it the bone planes are prepared sequentially. The prosthesis is then inserted in standard fashion. Results: Laboratory testing on dry bone and cadaveric models have confirmed that the registration process is now accurate. At the moment we are using a classical ICP algorithm to register the data points. For this test the Root Mean Square is 0.626 mm in a cadaveric model. This pinless anatomic registration can be achieved rapidly, if the initial siting points are accurately identified. Conclusion: The active constraint concept seems to be a safe and user friendly way of achieving robotic level accuracy with a human touch. Anatomic registration using the robot is accurate, and early clincal trials of total knee arthroplasty are encouraging