Introduction. Ability to accommodate increased range of motion is a design objective of many modern TKA prostheses. One challenge that any “high-flex friendly” prosthesis has to overcome is to manage the femorotibial contact stress at higher flexion angle, especially in the polyethylene tibial insert. When knee flexion angle increases, the femorotibial contact area tends to decrease thus the contact stress increases. For a high-flex design, considerations should be taken to control the contact stress to reduce the risk of early damage or failure on the tibial insert. This study evaluated the effect of femoral implant design on high flexion contact stress. Two prostheses from a same TKA family were compared – one as a conventional design and the other as a high-flex design. Methods. Two cruciate retaining (CR) prostheses from a same TKA product family were included in this study. The first is a conventional design for up to 125° of flexion (Optetrak CR, Exactech, USA). The second is a high-flex design for up to 145° of flexion (Logic CR, Exactech, USA). The high-flex design has a femoral component which has modified posterior condyle geometry (Figure 1), with the intent to increase femorotibial contact area and decrease contact stress at high flexion. Three sizes (sizes 1, 3, and 5) from each prosthesis line were included to represent the commonly used size spectrum. Contact stress was evaluated at 135° of flexion using finite element analysis (FEA). The CAD models were simplified and finite element models were created assuming all materials as linear elastic (Figure 2). For comparison purpose, a compressive force of 20% body weight was applied to the femoral component. The average body masses of sizes 1, 3 and 5 patients are 69.6 kg, 89.9 kg, and 106.3 kg based on the manufacture's clinical database. A nonlinear FEA solver was used to solve the simulation. Von Mises stress in the tibial insert was examined and compared between the two prostheses. Results. The high-flex design demonstrated lower tibial insert stresses compared to the conventional design, and the stress reduction is consistent across different sizes (Figure 3). The peak von Mises stress of the high-flex design was 8.6 MPa, 10.8 MPa, and 11.9 MPa for sizes 1, 3 and 5, representing a 40% to 60% decrease compared to those of the conventional design (14.3 MPa, 26.5 MPa, and 25.6 MPa respectively). Discussion/Conclusion. One limitation of the study was that no material nonlinearity was considered in the FEA, thus stress values above the yield strength of polyethylene could be over-estimated. However, as a qualitative comparison, the analysis demonstrated the effectiveness of the high-flex design on reducing tibial insert contact stress. Although the actual flexion angle of a CR TKA patient is not fully defined by the prosthesis and largely affected by the patient's anatomy and pre-operative range of motion, a lower contact stress at high flexion indicates a more forgiving mechanical structure and less risk for polyethylene damage when the patient is able to perform high flexion activities

Introduction

Conventional implant designs in total knee arthroplasty (TKA) are based on metal on UHMWPE bearing couples. Although this procedure is quite successful, early loosening is still a matter of concern. One of the causes for early failure is stress shielding, leading to loss of bone stock, periprosthetic bone fractures and eventually aseptic loosening of the component. The introduction of a polyetheretherketone (PEEK) on UHMWPE bearing couple could address this problem. With mechanical properties more similar to distal (cortical) bone it could allow stresses to be distributed more naturally in the distal femur. A potential adverse effect, however, is that the femoral component and the underlying cement mantle may be at risk of fracturing. Therefore, we analyzed the effect of a PEEK-Optima® femoral component on stress shielding and the integrity of the component and cement mantle, compared to a conventional Cobalt-Chromium (CoCr) alloy implant.

Aims

Periprosthetic femoral fractures (PFF) following total hip arthroplasty (THA) are devastating complications that are associated with functional limitations and increased overall mortality. Although cementless implants have been associated with an increased risk of PFF, the precise contribution of implant geometry and design on the risk of both intra-operative and post-operative PFF remains poorly investigated. A systematic review was performed to aggregate all of the PFF literature with specific attention to the femoral implant used.

Aims. In computer simulations, the shape of the range of motion (ROM) of a stem with a cylindrical neck design will be a perfect cone. However, many modern stems have rectangular/oval-shaped necks. We hypothesized that the rectangular/oval stem neck will affect the shape of the ROM and the prosthetic impingement. Methods. Total hip arthroplasty (THA) motion while standing and sitting was simulated using a MATLAB model (one stem with a cylindrical neck and one stem with a rectangular neck). The primary predictor was the geometry of the neck (cylindrical vs rectangular) and the main outcome was the shape of ROM based on the prosthetic impingement between the neck and the liner. The secondary outcome was the difference in the ROM provided by each neck geometry and the effect of the pelvic tilt on this ROM. Multiple regression was used to analyze the data. Results. The stem with a rectangular neck has increased internal and external rotation with a quatrefoil cross-section compared to a cone in a cylindrical neck. Modification of the cup orientation and pelvic tilt affected the direction of projection of the cone or quatrefoil shape. The mean increase in internal rotation with a rectangular neck was 3.4° (0° to 7.9°; p < 0.001); for external rotation, it was 2.8° (0.5° to 7.8°; p < 0.001). Conclusion. Our study shows the importance of attention to femoral implant design for the assessment of prosthetic impingement. Any universal mathematical model or computer simulation that ignores each stem’s unique neck geometry will provide inaccurate predictions of prosthetic impingement. Cite this article: Bone Joint Res 2021;10(12):780–789

The June 2023 Knee Roundup³⁶⁰ looks at: Cementless total knee arthroplasty is associated with early aseptic loosening in a large national database; Is cementless total knee arthroplasty safe in females aged over 75 years?; Could novel radiological findings help identify aseptic tibial loosening?; The Attune cementless versus LCS arthroplasty at introduction; Return to work following total knee arthroplasty and unicompartmental knee arthroplasty; Complications and downsides of the robotic total knee arthroplasty; Mid-flexion instability in kinematic alignment better with posterior-stabilized and medial-stabilized implants?; Patellar resurfacing does not improve outcomes in modern knees.

The success of a cementless Total Hip Arthroplasty (THA) depends not only on initial micromotion, but also on long-term failure mechanisms, e.g., implant-bone interface stresses and stress shielding. Any preclinical investigation aimed at designing femoral implant needs to account for temporal evolution of interfacial condition, while dealing with these failure mechanisms. The goal of the present multi-criteria optimization study was to search for optimum implant geometry by implementing a novel machine learning framework comprised of a neural network (NN), genetic algorithm (GA) and finite element (FE) analysis. The optimum implant model was subsequently evaluated based on evolutionary interface conditions. The optimization scheme of our earlier study [1] has been used here with an additional inclusion of an NN to predict the initial fixation of an implant model. The entire CAD based parameterization technique for the implant was described previously [1]. Three objective functions, the first two based on proximal resorbed Bone Mass Fraction (BMF) [1] and implant-bone interface failure index [1], respectively, and the other based on initial micromotion, were formulated to model the multi-criteria optimization problem. The first two objective functions, e.g., objectives f1 and f2, were calculated from the FE analysis (Ansys), whereas the third objective (f3) involved an NN developed for the purpose of predicting the post-operative micromotion based on the stem design parameters. Bonded interfacial condition was used to account for the effects of stress shielding and interface stresses, whereas a set of contact models were used to develop the NN for faster prediction of post-operative micromotion. A multi-criteria GA was executed up to a desired number of generations for optimization (Fig. 1). The final trade-off model was further evaluated using a combined remodelling and bone ingrowth simulation based on an evolutionary interface condition [2], and subsequently compared with a generic TriLock implant. The non-dominated solutions obtained from the GA execution were interpolated to determine the 3D nature of the Pareto-optimal surface (Fig. 2). The effects of all failure mechanisms were found to be minimized in these optimized solutions (Fig. 2). However, the most compromised solution, i.e., the trade-off stem geometry (TSG), was chosen for further assessment based on evolutionary interfacial condition. The simulation-based combined remodelling and bone ingrowth study predicted a faster ingrowth for TSG as compared to the generic design. The surface area with post-operative (i.e., iteration 1) ingrowth was found to be ∼50% for the TSG, while that for the TriLock model was ∼38% (Fig. 3). However, both designs predicted similar long-term ingrowth (∼89% surface area). The long-term proximal bone resorption (upto lesser trochanter) was found to be ∼30% for the TSG, as compared to ∼37% for the TriLock model. The TSG was found to be bone-preserving with prominent frontal wedge and rectangular proximal section for better rotational stability; features present in some recent designs. The optimization scheme, therefore, appears to be a quick and robust preclinical assessment tool for cementless femoral implant design. To view tables/figures, please contact authors directly

Summary. We report a large study of 331 patients at two years post operation who were prospectively randomised to receive either a rotating platform or a fixed bearing knee replacement of an otherwise identical design. Introduction. The mobile bearing total knee replacement was developed as there are theoretical benefits in that it may allow a better range of motion, better patella tracking and lower wear rates. This study was designed to see if these potential advantages are borne out in practice when using a cruciate retaining design. Methods. 331 patients undergoing TKR surgery were randomised to receive either a fixed bearing (170 patients) or a mobile bearing (161 patients). The femoral implant design was identical in the two groups. The tibial polyethylene bearing was either fixed in the metallic tibial tray (FB) or of rotating platform design (RP). All patients were assessed pre-operatively and at two years post-operatively using standard tools (Oxford, AKSS, Patellar Score) by independent nurse specialists. Results. The groups who received the FB and the RP implants have been assessed and their pre-operative to two year outcomes analysed with regard to the improvement in the range of motion (9.1 v. 10.2 degrees), Oxford Knee Score (-19.2 v. -17.6) and American Knee Society Knee and Function scores (51.3 v. 49.5 and 25.3 v. 23.6) at two years follow up. Conclusion. This large study shows that there is no statistical difference between a FB and a RP cruciate retaining TKR at two years post operation

Total hip replacement using porous-coated cobalt-chrome femoral implants designed for biological fixation has been evaluated in 307 patients after two years and in 89 patients after five years. Histological study of 11 retrieved specimens showed bone ingrowth in nine and fibrous tissue fixation in two. Fixation by bone ingrowth occurred in 93% of the cases in which a press fit of the stem at the isthmus was achieved, but in only 69% of those without a press fit. The clinical results at two years were excellent. The incidence of pain and limp was much lower when there was either a press fit of the stem or radiographic evidence of bone ingrowth. Factors such as age, sex, and the disease process did not influence the clinical results. Most cases showed only slight resorptive remodelling of the upper femur, but in a few cases with a larger, more rigid stem, more extensive bone loss occurred. The results after five years showed no deterioration with time. Fixation by the ingrowth of bone or of fibrous tissue both appeared to be stable, but bone ingrowth gave better clinical results

The objective of this present study was to determine the in vivo kinematic patterns for subjects implanted with a patellofemoral arthroplasty (PFA). Twenty subjects, all having a PFA, were studied (< 2 years post-op) under fluoroscopic surveillance to determine patellofemoral contact positions, sagittal plane, and medial/lateral translation using a skyline view. The patellofemoral contact patterns for each subject having a PFA was highly variable, 11.9 mm of translation. The average amount of patella rotation during the full flexion cycle was 26.3 degrees, while one subject experienced 48.6 degrees. The average amount of medial/lateral translation was 3.8 mm (5 > 5 mm). Five subjects experienced grater than 5 mm of motion. This was the first study to ever determine the in vivo kinematics for subjects having a PFA and the in vivo medial/lateral translation patterns of the patellofemoral joint. Subjects in this study experienced high variability and some abnormal rotational patterns. Most of the subjects who underwent PFA in this study had a previous history of subluxed or dislocated patella which affects the normal patella tracking, especially regarding tilting and translation. This tracking may also be directly affected by patellofemoral conformity, a consequence of femoral implant design. Finally, after PFA the patello-tibial tilt angle is influenced by the anteroposterior positioning of the femoral component. The results of this very first in vivo kinematic study may play an important role, not only for design consideration of patellofemoral replacement but also for surgical technique in order to obtain optimal implant positioning

Aims

The Exeter V40 cemented polished tapered stem system has demonstrated excellent long-term outcomes. This paper presents a systematic review of the existing literature and reports on a large case series comparing implant fractures between the Exeter V40 series; 125 mm and conventional length stem systems.

Trochlear geometry of modern femoral implants is designed for mechanical alignment (MA) technique for TKA. The biomechanical goal is to create a proximalised and more valgus trochlea to better capture the patella and optimize tracking. In contrast, Kinematic alignment (KA) technique for TKA respects the integrity of the soft tissue envelope and therefore aims to restore native articular surfaces, either femoro-tibial or femoro-patellar. Consequently, it is possible that current implant designs are not suitable for restoring patient specific trochlea anatomy when they are implanted using the kinematic technique, this could cause patellar complications, either anterior knee pain, instability or accelerated wear or loosening. The aim of our study is therefore to explore the extent to which native trochlear geometry is restored when the Persona. ®. implant (Zimmer, Warsaw, USA) is kinematically aligned. A retrospective study of a cohort of 15 patients with KA-TKA was performed with the Persona. ®. prosthesis (Zimmer, Warsaw, USA). Preoperative knee MRIs and postoperative knee CTs were segmented to create 3D femoral models. MRI and CT segmentation used Materialise Mimics and Acrobot Modeller software, respectively. Persona. ®. implants were laser scanned to generate 3D implant models. Those implant models have been overlaid on the 3D femoral implant model (generated via segmentation of postoperative CTs) to replicate, in silico, the alignment of the implant on the post-operative bone and to reproduce in the computer models the features of the implant lost due to CT metal artefacts. 3D models generated from post-operative CT and pre-operative MRI were registered to the same coordinate geometry. A custom written planner was used to align the implant, as located on the CT, onto the pre-operative MRI based model. In house software enabled a comparison of trochlea parameters between the native trochlea and the performed prosthetic trochlea. Parameters assessed included 3D trochlear axis and anteroposterior offset from medial facet, central groove, and lateral facet. Sulcus angle at 30% and 40% flexion was also measured. Inter and intra observer measurement variabilities have been assessed. Varus-valgus rotation between the native and prosthetic trochleae was significantly different (p<0.001), with the prosthetic trochlear groove being on average 7.9 degrees more valgus. Medial and lateral facets and trochlear groove were significantly understuffed (3 to 6mm) postoperatively in the proximal two thirds of the trochlear, with greatest understuffing for the lateral facet (p<0.05). The mean medio-lateral translation and internal-external rotation of the groove and the sulcus angle showed no statistical differences, pre and postoperatively. Kinematic alignment of Persona. ®. implants poorly restores native trochlear geometry. Its clinical impact remains to be defined

BACKGROUND. Trochlear geometry of modern femoral implants is designed for the mechanical alignment (MA) technique for Total Knee Arthroplasty (TKA). The biomechanical goal is to create a proximalised and more valgus trochlea to better capture the patella and optimize tracking. In contrast, Kinematic alignment (KA) technique for TKA respects the integrity of the soft tissue envelope and therefore aims to restore native articular surfaces, either femoro-tibial or femoro-patellar. Consequently, it is possible that current implant designs are not suitable for restoring patient specific trochlea anatomy when they are implanted using the kinematic technique. This could cause patellar complications, either anterior knee pain, instability or accelerated wear or loosening. The aim of our study is therefore to explore the extent to which native trochlear geometry is restored when the Persona. ®. implant (Zimmer, Warsaw, USA) is kinematically aligned. METHODS. A retrospective study of a cohort of 15 patients with KA-TKA was performed with the Persona. ®. prosthesis (Zimmer, Warsaw, USA). Preoperative knee MRIs and postoperative knee CTs were segmented to create 3D femoral models. MRI and CT segmentation used Materialise Mimics® and Acrobot Modeller® software, respectively. Persona. ®. implants were laser-scanned to generate 3D implant models. Those implant models have been overlaid on the 3D femoral implant model (generated via segmentation of postoperative CTs) to replicate, in silico, the alignment of the implant on the post-operative bone and to reproduce in the computer models the features of the implant lost due to CT metal artefacts. 3D models generated from post-operative CT and pre-operative MRI were registered to the same coordinate geometry. A custom written planner was used to align the implant, as located on the CT, onto the pre-operative MRI based model (figure 1). In house software enabled a comparison of trochlea parameters between the native trochlea and the performed prosthetic trochlea (figure 2). Parameters assessed included 3D trochlear axis and anteroposterior offset from medial facet, central groove, and lateral facet. Sulcus angle at 30% and 40% flexion was also measured. Inter and intra observer measurement variabilities have been assessed. RESULTS. Varus-valgus rotation between the native and prosthetic trochleae was significantly different (p<0.001), with the prosthetic trochlear groove being on average 7.9 degrees more valgus. Medial and lateral facets and trochlear groove were significantly understuffed (3 to 6mm) postoperatively in the proximal two thirds of the trochlear, with greatest understuffing for the lateral facet (p<0.05). The mean medio-lateral translation and internal-external rotation of the groove and the sulcus angle showed no statistical differences, pre and postoperatively (figure 3). CONCLUSION. Kinematic alignment of Persona. ®. implants poorly restores native trochlear geometry. The clinical impact of this finding remains to be defined. For figures/tables, please contact authors directly.

We describe two cases of fracture of Corin Taper-Fit stems used for cement-in-cement revision of congenital dysplasia of the hip. Both prostheses were implanted in patients in their 50s, with high offsets (+7.5 mm and +3.5 mm), one with a large diameter (48 mm) head and one with a constrained acetabular component. Fracture of the stems took place at nine months and three years post-operatively following low-demand activity. Both fractures occurred at the most medial of the two stem introducer holes in the neck of the prosthesis, a design feature that is unique to the Taper-Fit stem. We would urge caution in the use of these particular stems for cement-in-cement revisions.