INTRODUCTION. During TKA surgery, the usual goal is to achieve equal balancing between the lateral and medial side, which can be achieved by ligament releases or “pie crusting”. However little is known regarding a relationship between the balancing forces on the medial and lateral plateaus during TKA surgery, and the varus and valgus and rotational laxities when the TKA components are inserted. It seems preferable that the laxity after TKA is the same as for the normal intact knee. Hence the first aim of this study was to compare the laxity envelope of a native knee, with the same knee after TKA surgery. The second aim was to examine the relationship between the Varus-Valgus (VV) laxity and the contact forces on the tibial plateau. METHODS. A special rig that reproduced surgical conditions and fit onto an operating table was designed (Figure 1) (Verstraete et al. 2015). The rig allows application of a constant varus/valgus moment, and an internal-external (IE) torque. A series of heel push tests under these loading conditions were performed on 12 non-arthritic half semibodies hip-to-toe cadaveric specimens. Five were used for method development. To measure laxities, the flexion angle, the VV and the IE angle were measured using a navigation system. After testing the native knee, a TKA was performed using the Journey II BCS implant, the navigation assuring correct alignments. Soft tissue balancing was achieved by measuring compressive forces on the lateral and medial condyles with an instrumented tibial trial (Orthosensor, Dania Beach, Florida). At completion of the procedure, the laxity tests were repeated for VV and IE rotation and the contact forces on the tibial plateau were recorded, for the full range of flexion. RESULTS. The average of the varus-valgus and the IE laxity envelope is plotted for the native (yellow), the TKA (pink) and the overlap between the two (orange) (Figure 2). The average for six specimens of the contact force ratio (medial/medial+lateral force) during the varus and valgus test is plotted as a function of the laxity for each flexion angle (Figure 3). DISCUSSION. The Journey II implant replicated the VV laxity of the native knee except for up to 3 degrees more valgus in high flexion. For the IE, the TKA was equal in internal rotation, but up to 5 degrees more constrained in varus in mid range. Plotting contact force ratio against VV laxity (figure 3), as expected during the varus test the forces were clustered in a 0.85–0.95 ratio, implying predominant medial force with likely lateral lift-off. For the valgus test, the force ratio is more spread out, with all the values below 0.6. This could be due to the different stiffness of the MCL and LCL ligaments which are stressed during the VV test. During both tests the laxity increases progressively with flexion angle. Evidently the geometry knee reproduces more lateral laxity at higher flexion as in the anatomic situation. For figures/tables, please contact authors directly.

During TKA surgery, the usual goal is to achieve equal balancing between the lateral and medial side, which can be achieved by ligament releases or “pie crusting”. However little is known regarding a relationship between the balancing forces on the medial and lateral plateaus during TKA surgery, and the varus and valgus and rotational laxities when the TKA components are inserted. It seems preferable that the laxity after TKA is the same as for the normal intact knee. Hence the first aim of this study was to compare the laxity envelope of a native knee, with the same knee after TKA surgery. The second aim was to examine the relationship between the Varus-Valgus (VV) laxity and the contact forces on the tibial plateau. A special rig that reproduced surgical conditions and fit onto an operating table was designed (Verstraete et al. 2015). The rig allows application of a constant varus/valgus moment, and an internal-external (IE) torque. A series of heel push tests under these loading conditions were performed on 12 non-arthritic half semibodies hip-to-toe cadaveric specimens. Five were used for method development. To measure laxities, the flexion angle, the VV and the IE angle were measured using a navigation system. After testing the native knee, a TKA was performed using the Journey II BCS implant, the navigation assuring correct alignments. Soft tissue balancing was achieved by measuring compressive forces on the lateral and medial condyles with an instrumented tibial trial (Orthosensor, Dania Beach, Florida). At completion of the procedure, the laxity tests were repeated for VV and IE rotation and the contact forces on the tibial plateau were recorded, for the full range of flexion. The average of the varus-valgus and the IE laxity envelope is plotted for the native (yellow), the TKA (pink) and the overlap between the two (orange). The average for six specimens of the contact force ratio (medial/medial+lateral force) during the varus and valgus test is plotted as a function of the laxity for each flexion angle. The Journey II implant replicated the VV laxity of the native knee except for up to 3 degrees more valgus in high flexion. For the IE, the TKA was equal in internal rotation, but up to 5 degrees more constrained in varus in mid range. Plotting contact force ratio against VV laxity, as expected during the varus test the forces were clustered in a 0.85–0.95 ratio, implying predominant medial force with likely lateral lift-off. For the valgus test, the force ratio is more spread out, with all the values below 0.6. This could be due to the different stiffness of the MCL and LCL ligaments which are stressed during the VV test. During both tests the laxity increases progressively with flexion angle. Evidently the geometry knee reproduces more lateral laxity at higher flexion as in the anatomic situation

Aims. The goal was to evaluate tibiofemoral knee joint kinematics during stair descent, by simulating the full stair descent motion in vitro. The knee joint kinematics were evaluated for two types of knee implants: bi-cruciate retaining and bi-cruciate stabilized. It was hypothesized that the bi-cruciate retaining implant better approximates native kinematics. Methods. The in vitro study included 20 specimens which were tested during a full stair descent with physiological muscle forces in a dynamic knee rig. Laxity envelopes were measured by applying external loading conditions in varus/valgus and internal/external direction. Results. The laxity results show that both implants are capable of mimicking the native internal/external-laxity during the controlled lowering phase. The kinematic results show that the bi-cruciate retaining implant tends to approximate the native condition better compared to bi-cruciate stabilized implant. This is valid for the internal/external rotation and the anteroposterior translation during all phases of the stair descent, and for the compression-distraction of the knee joint during swing and controlled lowering phase. Conclusion. The results show a better approximation of the native kinematics by the bi-cruciate retaining knee implant compared to the bi-cruciate stabilized knee implant for internal/external rotation and anteroposterior translation. Whether this will result in better patient outcomes remains to be investigated. Cite this article: Bone Joint Res 2023;12(4):285–293

Preoperative ligament laxity can be characterized intraoperatively using digital robotic tensioners. Understanding how preoperative knee joint laxity affects preoperative and early post-operative patient reported outcomes (PROMs) may aid surgeons in tailoring intra-operative balance and laxity to optimize outcomes for specific patients. This study aims to determine if preoperative ligament laxity is associated with PROMs, and if laxity thresholds impact PROMs during early post-operative recovery. 106 patients were retrospectively reviewed. BMI was 31±7kg/m. 2. Mean age was 67±8 years. 69% were female. Medial and lateral knee joint laxity was measured intraoperatively using a digital robotic ligament tensioning device after a preliminary tibial resection. Linear regressions between laxity and KOOS12-function were performed in extension (10°), midflexion (45°), and flexion (90°) at preoperative, 6-week, and 3-month time points. Patients were separated into two laxity groups: ≥7 mm laxity and <7 mm laxity. Student's t-tests determined significant differences between laxity groups for KOOS12-function scores at all time points. Correlations were found between preoperative KOOS12-function and medial laxity in midflexion (p<0.001) and flexion (p<0.01). Patients with <7 mm of medial laxity had greater preoperative KOOS12-function scores compared to patients with ≥7 mm of medial laxity in extension (46.8±18.2 vs. 29.5±15.6, p<0.05), midflexion (48.4±17.8 vs. 32±16.1, p<0.001), and flexion (47.7±18.3 vs. 32.6±14.7, p<0.01). No differences in KOOS12-function scores were observed between medial laxity groups at 6-weeks or 3-months. All knees had <5 mm of medial laxity postoperatively. No correlations were found between lateral laxity and KOOS12-function. Patients with preoperative medial laxity ≥7 mm had lower preoperative PROMs scores compared to patients with <7 mm of medial laxity. No differences in PROMs were observed between laxity groups at 6 weeks or 3 months. Patients with excessive preoperative joint laxity achieve similar PROMs scores to those without excessive laxity after undergoing gap balancing TKA

Mechanical alignment (MA) in total knee arthroplasty (TKA), although considered the gold standard, reportedly has up to 25% of patients expressing post-operative dissatisfaction. Biomechanical outcomes following kinematic alignment (KA) in TKA, developed to restore native joint alignment, remain unclear. Without a clear consensus for the optimal alignment strategy during TKA, the purpose of this study was to conduct a paired biomechanical comparison of MA and KA in TKA by experimentally quantifying joint laxity and medial collateral ligament (MCL) strain. 14 bilateral native fresh-frozen cadaveric lower limbs underwent medially-stabilised TKA (GMK Sphere, Medacta, Switzerland) using computed CT-based subject-specific guides, with KA and MA performed on left and right legs, respectively. Each specimen was subjected to sensor-controlled mediolateral laxity tests. A handheld force sensor (Mark-10, USA) was used to generate an abduction-adduction moment of 10Nm at the knee at fixed flexion angles (0°, 30°, 60°, 90°). A digital image correlation system was used to compute the strain on the superficial medial collateral ligament. A six-camera optical motion capture system (Vicon MX+, UK) was used to acquire kinematics using a pre-defined CT-based anatomical coordinate system. A linear mixed model and Tukey's posthoc test were performed to compare native, KA and MA conditions (p<0.05). Unlike MA, medial joint laxity in KA was similar to the native condition; however, no significant difference was found at any flexion angle (p>0.08). Likewise, KA was comparable with the native condition for lateral joint laxity, except at 30°, and no statistical difference was observed. Although joint laxity in MA seemed lower than the native condition, this difference was significant only for 30° flexion (p=0.01). Both KA and MA exhibited smaller MCL strain at 0° and 30°; however, all conditions were similar at 60° and 90°. Medial and lateral joint laxity seemed to have been restored better following KA than MA; however, KA did not outperform MA in MCL strain, especially after mid-flexion. Although this study provides only preliminary indications regarding the optimal alignment strategy to restore native kinematics following TKA, further research in postoperative joint biomechanics for load bearing conditions is warranted

Inverse Kinematic Alignment (iKA) and Gap Balancing (GB) aim to achieve a balanced TKA via component alignment. However, iKA aims to recreate the native joint line versus resecting the tibia perpendicular to the mechanical axis. This study aims to compare how two alignment methods impact 1) gap balance and laxity throughout flexion and 2) the coronal plane alignment of the knee (CPAK). Two surgeons performed 75 robotic assisted iKA TKA's using a cruciate retaining implant. An anatomic tibial resection restored the native joint line. A digital joint tensioner measured laxity throughout flexion prior to femoral resection. Femoral component position was adjusted using predictive planning to optimize balance. After femoral resection, final joint laxity was collected. Planned GB (pGB) was simulated for all cases posthoc using a neutral tibial resection and adjusting femoral position to optimize balance. Differences in ML balance, laxity, and CPAK were compared between planned iKA (piKA) and pGB. ML balance and laxity were also compared between piKA and final (fiKA). piKA and pGB had similar ML balance and laxity, with mean differences <0.4mm. piKA more closely replicated native MPTA (Native=86.9±2.8°, piKA=87.8±1.8°, pGB=90±0°) and native LDFA (Native=87.5±2.7°, piKA=88.9±3°, pGB=90.8±3.5°). piKA planned for a more native CPAK distribution, with the most common types being II (22.7%), I (20%), III (18.7%), IV (18.7%) and V (18.7%). Most pGB knees were type V (28.4%), VII (37.8%), and III (16.2). fiKA and piKA had similar ML balance and laxity, however fiKA was more variable in midflexion and flexion (p<0.01). Although ML balance and laxity were similar between piKA and pGB, piKA better restored native joint line and CPAK type. The bulk of pGB knees were moved into types V, VII, and III due to the neutral tibial cut. Surgeons should be cognizant of how these differing alignment strategies affect knee phenotype

Introduction. Surgeons commonly resect additional distal femur during primary total knee arthroplasty (TKA) to correct a flexion contracture to restore range of motion and knee function. However, the effect of joint line elevation on the resulting TKA kinematics including frontal plane laxity is unclear. Thus, our goal was to quantify the effect of additional distal femoral resection on passive extension and mid-flexion laxity. Methods. Six computational knee models with capsular and collateral ligament properties specific to TKA were developed and implanted with a contemporary posterior-stabilized TKA. A 10° flexion contracture was modeled by imposing capsular contracture as determined by simulating a common clinical exam of knee extension and accounting for the length and weight of each limb segment from which the models were derived (Figure 1). Distal femoral resections of 2 mm and 4 mm were simulated for each model. The knees were then extended by applying the measured knee moments to quantify the amount of knee extension. The output data were compared with a previous cadaveric study using a two-sample two-tailed t-test (p<0.05) [1]. Subsequently, varus and valgus torques of ±10 Nm were applied as the knee was flexed from 0° to 90° at the baseline, and after distal resections of 2 mm, and 4 mm. Coronal laxity, defined as the sum of varus and valgus angulation in response to the applied varus and valgus torques, was measured at 30° and 45°of flexion, and the flexion angle was identified where the increase in laxity was the greatest with respect to baseline. Results. With 2 mm and 4 mm of distal femoral resection, the knee extended an additional 4°±0.5° and 8°±0.75°, respectively (Figure 2). No significant difference was found between the extension angle predicted by the six models and the results of the cadaveric study after 2 mm (p= 0.71) and 4 mm (p= 0.47). At 2 mm resection, mean coronal laxity increased by 3.1° and 2.7° at 30° and 45°of flexion, respectively. At 4 mm resection, mean coronal laxity increased by 6.5° and 5.5° at 30° and 45° of flexion, respectively (Figures 3a and 3b). The flexion angle corresponding to the greatest increase in coronal laxity for 2 mm of distal resection occurred at 22±7° of flexion with a mean increase in laxity of 4.0° from baseline. For 4 mm distal resection, the greatest increase in coronal laxity occurred at 16±6° of flexion with a mean increase in laxity of 7.8° from baseline. Conclusion. A TKA computational model representing a knee with preoperative flexion contracture was developed and corroborated measures from a previous cadaveric study [1]. While additional distal femoral resection in primary TKA increases passive knee extension, the consequent joint line elevation induced up to 8° of additional coronal laxity in mid-flexion. This additional midflexion laxity could contribute to midflexion instability; a condition that may require TKA revision surgery. Further studies are warranted to understand the relationship between joint line elevation, midflexion laxity, and instability. For any figures or tables, please contact the authors directly

Background. Lateral ankle instability is a common problem, but the precise role of the lateral ankle structures has not been accurately investigated. This study aimed to accurately investigate lateral ankle complex stability for the first time using a novel robotic testing platform. Method. A six degrees of freedom robot manipulator and a universal force/torque sensor were used to test 10 foot and ankle specimens. The system automatically defined the path of unloaded plantar/dorsi flexion. At four flexion angles: 20° dorsiflexion, neutral flexion, 20° and 40° of plantarflexion; anterior-posterior (90N), internal-external (5Nm) and inversion-eversion (8Nm) laxity were tested. The motion of the intact ankle was recorded first and then replayed following transection of the lateral retinaculum, Anterior Talofibular Ligament (ATFL) and Calcaneofibular Ligament (CFL). The decrease in force/torque reflected the contribution of the structure to restraining laxity. Data were analysed using repeated measures of variance and paired t-tests. Results. The ATFL was the primary restraint to anterior drawer (P< 0.01) and the CFL the primary restraint to inversion throughout range (P< 0.04), but with increased plantarflexion the ATFL's contribution increased. The ATFL had a significant role in resisting tibial external rotation, particularly at higher levels of plantarflexion, contributing 63% at 40° (P< 0.01). The CFL provided the greatest resistance to external tibial rotation, 22% at 40° plantarflexion (P< 0.01). The extensor retinaculum and skin did not offer significant restraint in any direction tested. Conclusion. This study shows accurately for the first time the significant role the ATFL and CFL have in rotational ankle stability. This significant loss in rotational stability may have implications in the aetiology of osteophyte formation and early degenerative changes in patients with chronic ankle instability. This is the first time the role of the lateral ankle complex has been quantified using a robotic testing platform

The aim of an anterior cruciate ligament (ACL) reconstruction is to regain functional stability of the knee following ACL injury, ideally allowing patients to return to their pre-injury level of activity. The purpose of this study was to assess clinical, functional and patient-reported outcomes following primary ACL reconstruction with hamstring autograft. A prospective case-series design (n=1610) was used to gather data on post-operative ACL graft laxity, functional testing performance and scores on the ACL quality of life (ACL-QOL) questionnaire. Demographic data were collected for all patients. Post-operative ACL laxity assessment using the Lachman and Pivot-shift tests was completed independently on each patient by a physiotherapist and an orthopaedic surgeon at the 6-, 12- and 24-months post-operative appointments. A battery of functional tests was also assessed including single leg Bosu balance, and 4 single-leg hop tests. The hop tests provided a comparative assessment of limb-to-limb function. Patients completed the ACL-QOL at all time points. The degree and frequency of post-operative laxity was calculated. A Spearman's rank correlation matrix was undertaken to assess for relationships between post-operative laxity, functional test performance, and the ACL-QOL scores. A linear regression model was used to assess for relationships between the ACL-QOL scores, as well as the functional testing results, and patient demographic factors. ACLR patients were 55% male, with a mean age of 29.7 years (SD=10.4), mean BMI of 25 (SD=3.9), and mean Beighton score of 3.3 (SD=2.5). At clinical assessment 2-years post-operatively, 20.6% of patients demonstrated a positive Lachman test and 7.7% of patients demonstrated a positive Pivot-shift test. The mean ACL-QOL score was 28.6/100 (SD=13.4) pre-operatively, 58.2/100 (SD=17.6) at 6-months, 71.8/100 (SD=18.1) at 12-months, and 77.4/100 (SD=19.2) at 24-months post-operative. Functional tests assessing operative to non-operative limb performance demonstrated that patients were continuing to improve up to the 24-month mark, with limb symmetry indices ranging from 96.6–103.1 for the single-leg hop tests. Spearman's correlation coefficient demonstrated a significant relationship between the presence of ACL graft laxity and ACL-QOL score at 12- and 24-months post-operative (p < 0 .05). Functional performance on the single leg balance and single-leg hop tests demonstrated significant correlations to the 6-, 12- and 24-month ACL-QOL scores (p < 0 .05). There was no statistically significant correlation between the functional testing results and the presence of ACL graft laxity. This study demonstrated that up to 20.6% of patients had clinically measurable graft laxity 2-years after ACLR. In this cohort, patients with graft laxity demonstrated lower ACL-QOL scores, but did not demonstrate lower functional testing performance. Patient-reported ACL-QOL scores improved significantly at each time point following ACLR, and functional performance continued to improve up to 2-years after surgery. The ACL-QOL score was strongly correlated to the patient's ability to perform single-limb functional tests, indicating that the ACL-QOL score accurately predicted level of function

The primary purpose of this study was to assess whether patients presenting with clinical graft laxity following primary anatomic anterior cruciate ligament (ACL) reconstruction using hamstring autograft reported a significant difference in disease-specific quality-of-life (QOL) as measured by the ACL-QOL questionnaire. Clinical ACL graft laxity was assessed in a cohort of 1134/1436 (79%) of eligible patients using the Lachman and Pivot-shift tests pre-operatively and at 12- and 24-months following ACL reconstruction. Post-operative ACL laxity was assessed by an orthopaedic surgeon and a physical therapist who were blinded to each other's examination. If there was a discrepancy between the clinical examination findings from these two assessors, then a third impartial examiner assessed the patient to ensure a grading consensus was reached. Patients completed the ACL-QOL questionnaire pre-operatively, and 12- and 24-months post-operatively. Descriptive statistics were used to assess patient demographics, rate of post-operative ACL graft laxity, surgical failures, and ACL-QOL scores. A Spearman rho correlation coefficient was utilised to assess the relationships between ACL-QOL scores and the Lachman and Pivot-shift tests at 24-months post-operative. An independent t-test was used to determine if there were differences in the ACL-QOL scores of subjects who sustained a graft failure compared to the intact graft group. ACL-QOL scores and post-operative laxity were assessed using a one-way analysis of variance (ANOVA). There were 70 graft failures (6.17%) in the 1134 patients assessed at 24-months. A total of 226 patients (19.9%) demonstrated 24-months post-operative ACL graft laxity. An isolated positive Lachman test was assessed in 146 patients (12.9%), an isolated positive Pivot-shift test was apparent in 14 patients (1.2%), and combined positive Lachman and Pivot-shift tests were assessed in 66 patients (5.8%) at 24-months post-operative. There was a statistically significant relationship between 24-month post-operative graft laxity and ACL-QOL scores (p < 0.001). Specifically, there was a significant correlation between the ACL-QOL and the Lachman test (rho = −0.20, p < 0.001) as well as the Pivot-shift test (rho = −0.22, p < 0.001). There was no significant difference between the scores collected from the graft failure group prior to failure occurring (mean = 74.38, SD = 18.61), and the intact graft group (mean = 73.97, SD = 21.51). At 24-months post-operative, the one-way ANOVA demonstrated a statistically significant difference between the ACL-QOL scores of the no laxity group (mean = 79.1, SD = 16.9) and the combined positive Lachman and Pivot-shift group (mean = 68.5, SD = 22.9), (p = 0, mean difference = 10.6). Two-years post ACL reconstruction, 19.9% of patients presented with clinical graft laxity. Post-operative graft laxity was significantly correlated with lower ACL-QOL scores. The difference in ACL-QOL scores for patients with an isolated positive Lachman or Pivot-shift test did not meet the threshold of a clinically meaningful difference. Patients with clinical laxity on both the Lachman and Pivot-shift tests demonstrated the lowest patient-reported ACL-QOL scores, and these results exceeded the minimal clinically important difference

Introduction. Surgeons commonly resect additional distal femur during primary total knee arthroplasty (TKA) to correct a flexion contracture. However, the effect of joint line proximalization on TKA kinematics is unclear. Thus, our goal was to quantify the effect of additional distal femoral resection on knee extension and mid-flexion laxity. Methods. Six computational knee models with TKA-specific capsular and collateral ligament properties were implanted with a contemporary posterior-stabilized TKA. A 10° flexion contracture was modeled to simulate a capsular contracture. Distal femoral resections of +2 mm and +4 mm were simulated for each model. The knees were then extended under standardized torque to quantify additional knee extension achieved. Subsequently, varus and valgus torques of ±10 Nm were applied as the knee was flexed from 0° to 90° at the baseline, +2 mm, and +4 mm distal resections. Coronal laxity, defined as the sum of varus and valgus angulation with respective torques, was measured at mid-flexion. Results. With +2 mm and +4 mm of distal femoral resection, the knee extended an additional 4°±0.5° and 8°±0.75°, respectively. At 30° and 45°of flexion, baseline laxity averaged 4.8° and 5.0°, respectively. At +2 mm resection, mean coronal laxity increased by 3.1° and 2.7° at 30° and 45°of flexion, respectively. At +4 mm resection, mean coronal laxity increased by 6.5° and 5.5° at 30° and 45° of flexion, respectively. Maximal increased coronal laxity for a +4 mm resection occurred at a mean 16° (range, 11–27°) of flexion with a mean increased laxity of 7.8° from baseline. Conclusion. While additional distal femoral resection in primary TKA increases knee extension, the consequent joint line elevation induces up to 8° of coronal laxity in mid-flexion in this computational model. As such, posterior capsular release prior to resecting additional distal femur to correct a flexion contracture should be considered

Introduction. When performing total knee arthroplasty (TKA), surgeons often utilize a posterior-stabilized (PS) design which compensates for the loss of the posterior cruciate ligament (PCL). These designs attempt to replicate normal knee kinematics and loading using a cam and post to provide posterior restraint of the tibia during flexion. However, these designs may not be able to compensate for the increase in flexion space or the inherent loss of coronal stability after PCL release compared to a cruciate retaining (CR) design. This study aimed to compare stability of PS and CR TKA designs by assessing laxity in three planes. Methods. The specimens utilized in this study were lower extremities from fresh cadavers of donors who had previously undergone a total knee replacement (Medical Education and Research Institute (Memphis, TN) and Restore Life USA (Johnson City, TN)). IRB approval was obtained prior to performing the study. Twenty-three knee specimens (8 left, 15 right) were retrieved and all skin, subcutaneous tissue and muscle was removed. The femur and tibia were cut transversely 180 mm superior and inferior to the knee joint line, respectively, and specimens were mounted in a custom knee testing machine. Specimens were tested with the knee joint at full extension and at 30, 60, and 90 degrees of flexion. Laxity was assessed at 1.5 Nm of internal and external torque and 10 Nm varus and valgus torque, as well as a 35 N anterior and posterior force. Laxity was expressed as degrees of tibial displacement in the coronal plane under a varus/valgus torque and degrees of displacement in the transverse plane under an internal/external torque, as well as mm of anterior or posterior displacement. TKA components were retrieved to determine PS or CR design and grouped accordingly. Results. Of the 23 implants, 10 were PS designs and 13 were CR. PS posterior laxity was 1 mm greater in full extension (p = 0.02, Figure 1), and PS varus laxity increased by 6 degrees at 90 degrees of flexion over CR laxity (p = 0.04, Figure 3). Varus to valgus laxity range of PS knees was greater than CR knees for all flexion angles. PS external rotational laxity at 90 degrees of flexion was greater than that of CR laxity by 7 degrees (p = 0.02, Figure 2). Discussion. Results indicate significant laxity differences between PS and CR designs in both full extension and 90 degrees of flexion. PS designs have decreased coronal stability compared to CR, but appear to mimic AP constraint in midflexion and flexion. Mihalko et al. (2000) showed that loss of the PCL during TKA leads to a decrease in coronal stability, which is confirmed here. The post and cam mechanism of the PS designs restores AP stability during flexion but does not restore this coronal stability. These results may be limited by variations in implant design

Aims. It is unknown whether gap laxities measured in robotic arm-assisted total knee arthroplasty (TKA) correlate to load sensor measurements. The aim of this study was to determine whether symmetry of the maximum medial and lateral gaps in extension and flexion was predictive of knee balance in extension and flexion respectively using different maximum thresholds of intercompartmental load difference (ICLD) to define balance. Methods. A prospective cohort study of 165 patients undergoing functionally-aligned TKA was performed (176 TKAs). With trial components in situ, medial and lateral extension and flexion gaps were measured using robotic navigation while applying valgus and varus forces. The ICLD between medial and lateral compartments was measured in extension and flexion with the load sensor. The null hypothesis was that stressed gap symmetry would not correlate directly with sensor-defined soft tissue balance. Results. In TKAs with a stressed medial-lateral gap difference of ≤1 mm, 147 (89%) had an ICLD of ≤15 lb in extension, and 112 (84%) had an ICLD of ≤ 15 lb in flexion; 157 (95%) had an ICLD ≤ 30 lb in extension, and 126 (94%) had an ICLD ≤ 30 lb in flexion; and 165 (100%) had an ICLD ≤ 60 lb in extension, and 133 (99%) had an ICLD ≤ 60 lb in flexion. With a 0 mm difference between the medial and lateral stressed gaps, 103 (91%) of TKA had an ICLD ≤ 15 lb in extension, decreasing to 155 (88%) when the difference between the medial and lateral stressed extension gaps increased to ± 3 mm. In flexion, 47 (77%) had an ICLD ≤ 15 lb with a medial-lateral gap difference of 0 mm, increasing to 147 (84%) at ± 3 mm. Conclusion. This study found a strong relationship between intercompartmental loads and gap symmetry in extension and flexion measured with prostheses in situ. The results suggest that ICLD and medial-lateral gap difference provide similar assessment of soft-tissue balance in robotic arm-assisted TKA. Cite this article: Bone Jt Open 2021;2(11):974–980

Navigation systems are able to measure very accurately the movement of bones, and consequently the knee laxity, which is a movement of the tibia under the femur. These systems might help measuring the knee laxity during the implantation of a total (TKR) or a unicompartmental (UKR) knee replacement. 20 patients operated on for TKR (13 cases) or UKR (7 cases) because of primary varus osteoarthritis have been analyzed. Pre-operative examination involved varus and valgus stress X-rays at 0 and 90° of knee flexion. The intra-operative medial and lateral laxity was measured with the navigation system at the beginning of the procedure and after prosthetic implantation. Varus and valgus stress X-rays were repeated after 6 weeks. X-ray and navigated measurements before and after knee replacement were compared with a paired Wilcoxon test at a 0.05 level of significance. The mean pre-operative medial laxity in extension was 2.3° (SD 2.3°). The mean pre-operative lateral laxity in extension was 5.6° (SD 5.1°). The mean pre-operative medial laxity in flexion was 2.2° (SD 1.9°). The mean pre-operative lateral laxity in flexion was 6.7° (SD 6.0°). The mean intra-operative medial laxity in extension at the beginning of the procedure was 3.6° (SD 1.7°). The mean intra-operative lateral laxity in extension at the beginning of the procedure was 3.0° (SD 1.3°). The mean intra-operative medial laxity in flexion at the beginning of the procedure was 1.9° (SD 2.6°). The mean intra-operative lateral laxity in flexion at the beginning of the procedure was 3.5° (SD 2.7°). The mean intra-operative medial laxity in extension after implantation was 2.1° (SD 0.9°). The mean intra-operative lateral laxity in extension after implantation was 1.9° (SD 1.1°). The mean intra-operative medial laxity in flexion after implantation was 1.9° (SD 2.5°). The mean intra-operative lateral laxity in flexion after implantation was 3.0° (SD 2.8°). The mean post-operative medial laxity in extension was 2.4° (SD 1.1°). The mean post-operative lateral laxity in extension was 2.0° (SD 1.7°). The mean post-operative medial laxity in flexion was 4.4° (SD 3.3°). The mean post-operative lateral laxity in flexion was 4.7° (SD 3.2°). There was a significant difference between navigated and radiographic measurements for the pre-operative medial laxity in extension (mean = 1.4° – p = 0.005), the pre-operative lateral laxity in extension (mean = 2.6° – p = 0.01), the pre-operative lateral laxity in flexion (mean = 3.3° – p = 0.005). There was no significant difference between navigated and radiographic measurements for the pre-operative medial laxity in flexion (mean = 0.3° – p = 0.63). There was a significant difference between navigated and radiographic measurements for the postoperative medial laxity in flexion (mean = 2.5° – p = 0.004). There was no significant difference between navigated and radiographic measurements for the postoperative medial laxity in extension (mean = 0.3° – p = 0.30), the post-operative lateral laxity in extension (mean = 0.2° – p = 0.76), the post-operative lateral laxity in flexion (mean = 1.7° – p = 0.06). These differences were less than 2 degrees in most of the cases, and then considered as clinically irrelevant. The navigation system used allowed measuring the medial and lateral laxity before and after TKR. This measurement was significantly different from the radiographic measurement by stress X-rays for pre-operative laxity, but not statistically different from the radiographic measurement by stress X-rays for post-operative laxity. The differences were mostly considered as clinically irrelevant. The navigated measurement of the knee laxity can be considered as accurate. The navigated measurement is valuable information for balancing the knee during TKR. The reproducibility of this balancing might be improved due to a more objective assessment

Introduction: Computer navigation in total knee arthroplasty (TKA) may assist the surgeon with precise information about ligament tension and varus/valgus alignment throughout the complete range of motion, but there is only little information about how much ligament laxity is needed and how much laxity is too much. In the current study we measured the mechanical axis and opening of the joint at different time points, in different degrees of knee flexion and with varus and valgus stress during the procedure of computer navigated TKA. Methods: Forty-nine consecutive patients underwent a MIS computer navigated TKA. With the Stryker Knee Navigation System varus/valgus alignment and distraction/compression was measured in 0°, 45° and 90° of knee flexion immediate after digitalization of the knee and after fascial closure. Values were noted in a neutral position and with maximal varus and maximal valgus stress applied. Patients with posterior stabilized implants were compared to those with cruciate retaining implants. Patients with preoperative varus malalignment or valgus malalignment were compared to patients with straight preoperative mechanical axes. Results: At the beginning of the operative procedure the mean mechanical alignment was 1.9° varus at 0° knee flexion, 1.5° varus at 45° knee flexion and 1.5° varus at 90° knee flexion. Patients showed a mean mediolateral joint opening of 6.1° at 0° knee flexion, 5.9° at 45° knee flexion and 4.5° at 90° knee flexion. After implantation of the knee prosthesis and fascial closure mechanical alignment was 0.3° varus at 0° knee flexion, 0° varus at 45° knee flexion and 0.2° varus at 90° knee flexion. Mean joint laxity was 3.4° at 0° knee flexion, 3.1° at 45° knee flexion and 2.3° at 90° knee flexion. There was more lateral than medial joint opening postoperatively in 45° and 90° knee flexion regardless of the prosthesis type implanted. Preoperative varus and valgus malalignment could be reduced to values identical with those patients with straight preoperative mechanical axes. Discussion: Mean varus/valgus laxity after implantation of a MIS computer navigated TKA was lower than prior to prosthesis implantation. Varus/valgus laxity of an approximate total range of 1.5°–2° can be achieved at all measured degrees of knee flexion and seems to be the ideal laxity for TKA. Computer navigation in TKA can consistently reduce preoperative varus/valgus malalignment to a level comparable to patients with preoperatively normal mechanical axes. More lateral joint opening is found before as well as after implantation of the prosthesis in 45° and 90° of knee flexion. The type of prosthesis implanted seems not to effect postoperative joint laxity

Postero-lateral rotator instability (PLRI) is the most common pattern of recurrent elbow instability. Unfortunately, current imaging to aid PLRI diagnosis is limited. We have developed an ultrasound (US) technique to measure ulnohumeral joint gap with and without stress of the lateral ulnocollateral ligament. We sought to define lateral ulnohumeral joint gap measurements in the resting and stressed state to provide insight into how US may aid diagnosis of PLRI. Sixteen elbows were evaluated in eight healthy volunteers. Lateral ulnohumeral gap was measured on US in the resting position and with posterolateral drawer stress test maneuver applied. Joint laxity was calculated as the difference between stress and rest conditions. Measurements were performed by two independent readers with comparison performed between stress and rest positions. A highly significant difference in ulnohumeral gap was seen between stress and rest conditions (Reader 1: p < 0 .0001 and Reader 2: p=0.0002) with median values of 2.93 mm and 2.50 mm at rest and 3.92 mm and 3.40 mm at stress for Reader 1 and 2 respectively. Median joint laxity was 1.02 mm and 0.74 mm respectively for each reader. Correlation and agreement between readers was good. This study provides key new insight into use of US for diagnosis as PLRI as it defines normal ulnohumeral distances and demonstrates widening when applying the posterolateral drawer stress maneuver. Further evaluation of this technique is required in patients with PLRI

Introduction. Total knee arthroplasty (TKA) is a successful procedure for end stage arthritis of the knee that is being performed on an exponential basis year after year. Most surgeons agree that soft tissue balancing of the TKA is a paramount to provide a successful TKA. We utilized a set of retrieved lower extremities with an existing TKA to measure the laxity of the knee in all three planes to see if wear scores of the implants correlated to the laxity measured. This data has never been reported in the literature. Methods. IRB approval was obtained for the local retrieval program. Each specimen was retrieved after removing the skin, subcutaneous tissue and muscle from mid thigh to mid tibia. The femur, tibia and fibula were then transversely cut to remove the specimen for testing. Each specimen was then imaged using a flouroscopic imaging unit (OEC, Inc) in the AP, Lateral and sunrise views. These images were used to analyze whether there were any signs of osteolysis. Each specimen was mounted into a custom knee testing machine (Little Rock AR). Each specimen then was tested at full extension, 30, 60, and 90 degrees of flexion. At each flexion angle the specimen was subjected to a 10Nm varus and valgus torque, a 1.5Nm internal and external rotational torque and a 35N anterior and posterior directed force. Each specimen's implants were removed to record manufacturer and lot numbers. Polyethylene damage scores (Hood et al. JBMR 1983) were then calculated in the medial, lateral and backside of the polyethylene insert as well as on the medial and lateral femoral condyle. (Figure 1) Correlation coefficients were then calculated to show any relationship with soft tissue balancing in all three planes and wear scores. Results. No correlation > 0.4 existed for any surface damage on the polyethylene or femoral condyle to laxity in any plane (Figure 2). The highest correlations were found with backside wear (0.5) to internal and external rotational laxity. Two thirds of the specimens had more varus than valgus laxity in the coronal plane (p=0.03). Discussion/Conclusion. This is the first report of necropsy obtained retrievals where the soft tissue laxity of the knee was recorded. Although small numbers with different implant types the data shows that limited correlation exists between implant surface damage and increased laxity. The strongest correlation we found was backside wear to transverse plane laxity in flexion and extension, but this most likely is related to locking mechanism design. It seems in this set of implants that the soft tissue laxity did not affect implant bone interfaces as all were over 10 years from surgery. For figures/tables, please contact authors directly.

The function of the knee joint is to allow for locomotion and is comprised of various bodily structures including the four major ligaments; medial collateral ligament (MCL), lateral collateral ligament (LCL), anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). The primary function of the ligaments are to provide stability to the joint. The knee is prone to injury as a result of osteoarthritis as well as ligamentous and meniscal lesions. Furthermore, compromised joint integrity due to ligamentous injury may be a result of direct and indirect trauma, illness, occupational hazard as well as lifestyle. A device capable of non-invasively determining the condition of the ligaments in the knee joint would be a useful tool to assist the clinician in making a more informed diagnosis and prognosis of the injury. Furthermore, the device would potentially reduce the probability of a misdiagnosis, timely diagnosis and avoidable surgeries. The existing Laxmeter prototype (UK IPN: GB2520046) is a Stress Radiography Device currently limited to measuring the laxity of the MCL and LCL at multiple fixed degrees of knee flexion. Laxity refers to the measure of a ligament's elasticity and stiffness i.e. the condition of the ligament, by applying a known load (200N) to various aspects of the proximal tibial and thereby inducing tibial translation. The extent of translation would indicate the condition of the ligament. The Laxmeter does not feature a load applying component as of yet, however, it allows for the patient to be in the most comfortable and ideal position during radiographic laxity measurement testing. The entire structure is radiolucent and attempts to address the limitations of existing laxity measurement devices, which includes: excessive radiation exposure to the radiographic assistant, little consideration for patient ergonomics and restrictions to cruciate or collateral ligament laxity measurements. The study focusses on further developing and modifying the Laxmeter to allow for: the laxity measurement of all four major ligaments of the knee joint, foldability for improved storage and increased structural integrity. Additionally, a load applicator has been designed as an add-on to the system thereby making the Laxmeter a complete Stress Radiography Device. Various materials including Nylon, Polycarbonate, Ultra High Molecular Weight Polyethylene (UHMWPE) – PE 1000, and Acetal/ POM were tested, using the Low Dose X-ray (Lodox) scanner, to determine their radiolucency. All materials were found to be radiolucent enough for the manufacture of the Laxmeter structure as well as the load applicator in order to identify and measure the translation of the tibia with respect to the stationary femur. The Laxmeter allows for the measurement of the laxity of the MCL and LCL at multiple fixed degrees of flexion by providing the ideal patient position for testing. The next iteration of the device will present an affordable and complete Stress Radiography Device capable of measuring the laxity of all four major ligaments of the knee joint at multiple fixed degrees of flexion. Future work would include aesthetic considerations as well as an investigation into carbon-fibre-reinforced plastics

Introduction. Total knee arthroprasty (TKA) is an excellent treatment with osteoarthritis of the knee joint. The acquisition of joint stability after TKA is one of the most important factors to improve the patient's quality of life. Deep flexion of knee joint is often demanded in daily life, and stability in flexed knee position is also important. But there were few papers reporting about laxity in flexed knee position. This study aimed to analyze influence of pre-operative alignment on post-operative varus-valgus joint laxity in TKA. We investigated the varus-valgus laxity of knee joint throughout flexion intra-operatively before and after prosthetic implantation. Methods. A total of 20 knees underwent TKA using posterior-stabilised (PS) type component by the measured resection method were included in this study. The varus-valgus joint laxity of knee was measured using an intra-operative navigation system at every 10 ° throughout the range of movement under general anesthesia. We examined the correlations between the pre-operative femorotibial angle (FTA) and varus-valgus joint laxity by method of least squeres. We divided the patients group into two populations according to pre-operative FTA. Large FTA group had more than or equal to 186 °of pre-operative FTA. Small FTA group had less than 186 °pre-operative FTA. T- test was performed between those populations. Result. After TKA, mean FTA improved from 189.15 °(SD = 5.87 SD: Standard Deviation) to 172.65 °(SD = 1.59). All of patients were improved in the Knee Society Score (KSS) and range of motion (ROM) (Fig 1). There were significant positive correlations between the pre-operative FTA and varus-valgus joint laxity in flexion of 90 °(CC = 0.48, P < 0.05 CC: Correlation Coefficient P: probability value), 100 °(CC = 0.57, P < 0.01), 110 °(CC = 0.55, P < 0.05), and 120 °(CC = 0.57, P < 0.01). In the large FTA group, the varus-valgus joint laxities were larger than that of small FTA group in initial flexed position before TKA (Fig 2), whereas the varus-valgus joint laxities were larger in flexed position after TKA (Fig 3). Discussion. Our results showed that in patients who had large FTA and were underwent TKA using PS type component by the measured resection method, they had large varus-valgus joint laxities in flexed knee position. There is a possibility that the increase of laxity in the flexed knee position was due to acquisition of stability with releasing of medial collateral ligament in the extended knee position not but in flexed knee position. In this study we demonstrated correlations between the pre-operative FTA and varus-valgus joint laxity in flexed knee position. In the further study, we would like to investigate how the increasing laxities in the flexion knee position affect the clinical symptoms

This study aims to create a novel computational workflow for frontal plane laxity evaluation which combines a rigid body knee joint model with a non-linear implicit finite-element model wherein collateral ligaments are anisotropically modelled using subject-specific, experimentally calibrated Holzpfel-Gasser-Ogden (HGO) models. The framework was developed based on CT and MRI data of three cadaveric post-TKA knees. Bones were segmented from CT-scans and modelled as rigid bodies in a multibody dynamics simulation software (MSC Adams/view, MSC Software, USA). Medial collateral and lateral collateral ligaments were segmented based on MRI-scans and are modelled as finite elements using the HGO model in Abaqus (Simulia, USA). All specimens were submitted varus/valgus loading (0-10Nm) while being rigidly fixed on a testing bench to prevent knee flexion. In subsequent computer simulations of the experimental testing, rigid bodies kinematics and the associated soft-tissue force response were computed at each time step. Ligament properties were optimised using a gradient descent approach by minimising the error between the experimental and simulation-based kinematic response to the applied varus/valgus loads. For comparison, a second model was defined wherein collateral ligaments were modelled as nonlinear no-compression spring elements using the Blankevoort formulation. Models with subject-specific, experimentally calibrated HGO representations of the collateral ligaments demonstrated smaller root mean square errors in terms of kinematics (0.7900° +/− 0.4081°) than models integrating a Blankevoort representation (1.4704° +/− 0.8007°). A novel computational workflow integrating subject-specific, experimentally calibrated HGO predicted post-TKA frontal-plane knee joint laxity with clinically applicable accuracy. Generally, errors in terms of tibial rotation were higher and might be further reduced by increasing the interaction nodes between the rigid body model and the finite element software. Future work should investigate the accuracy of resulting models for simulating unseen activities of daily living