Load transfer in bone after partial, multi-compartmental, and total knee arthroplasty.

Introduction: Arthroplasty-associated bone loss remains a clinical problem: stiff metallic implants disrupt load transfer to bone and, hence, its remodeling stimulus. The aim of this research was to analyze how load transfer to bone is affected by different forms of knee arthroplasty: isolated partial knee arthroplasty (PKA), compartmental arthroplasty [combined partial knee arthroplasty (CPKA), two or more PKAs in the same knee], and total knee arthroplasty (TKA). Methods: An experimentally validated subject-specific finite element model was analyzed native and with medial unicondylar, lateral unicondylar, patellofemoral, bi-unicondylar, medial bicompartmental, lateral bicompartmental, tricompartmental, and total knee arthroplasty. Three load cases were simulated for each: gait, stair ascent, and sit-to-stand. Strain shielding and overstraining were calculated from the differences between the native and implanted states. Results: For gait, the TKA femoral component led to mean strain shielding (30%) more than three times higher than that of PKA (4%-7%) and CPKA (5%-8%). Overstraining was predicted in the proximal tibia (TKA 21%; PKA/CPKA 0%-6%). The variance in the distribution for TKA was an order of magnitude greater than for PKA/CPKA, indicating less physiological load transfer. Only the TKA-implanted femur was sensitive to the load case: for stair ascent and gait, almost the entire distal femur was strain-shielded, whereas during sit-to-stand, the posterior femoral condyles were overstrained. Discussion: TKA requires more bone resection than PKA and CPKA. These finite element analyses suggest that a longer-term benefit for bone is probable as partial and multi-compartmental knee procedures lead to more natural load transfer compared to TKA. High-flexion activity following TKA may be protective of posterior condyle bone resorption, which may help explain why bone loss affects some patients more than others. The male and female bone models used for this research are provided open access to facilitate future research elsewhere.

The risk of tibial eminence avulsion fracture with bi-unicondylar knee arthroplasty : a finite element analysis.

AIMS: The aim of this study was to determine the risk of tibial eminence avulsion intraoperatively for bi-unicondylar knee arthroplasty (Bi-UKA), with consideration of the effect of implant positioning, overstuffing, and sex, compared to the risk for isolated medial unicondylar knee arthroplasty (UKA-M) and bicruciate-retaining total knee arthroplasty (BCR-TKA). METHODS: Two experimentally validated finite element models of tibia were implanted with UKA-M, Bi-UKA, and BCR-TKA. Intraoperative loads were applied through the condyles, anterior cruciate ligament (ACL), medial collateral ligament (MCL), and lateral collateral ligament (LCL), and the risk of fracture (ROF) was evaluated in the spine as the ratio of the 95th percentile maximum principal elastic strains over the tensile yield strain of proximal tibial bone. RESULTS: Peak tensile strains occurred on the anterior portion of the medial sagittal cut in all simulations. Lateral translation of the medial implant in Bi-UKA had the largest increase in ROF of any of the implant positions (43%). Overstuffing the joint by 2 mm had a much larger effect, resulting in a six-fold increase in ROF. Bi-UKA had ~10% increased ROF compared to UKA-M for both the male and female models, although the smaller, less dense female model had a 1.4 times greater ROF compared to the male model. Removal of anterior bone akin to BCR-TKA doubled ROF compared to Bi-UKA. CONCLUSION: Tibial eminence avulsion fracture has a similar risk associated with Bi-UKA to UKA-M. The risk is higher for smaller and less dense tibiae. To minimize risk, it is most important to avoid overstuffing the joint, followed by correctly positioning the medial implant, taking care not to narrow the bone island anteriorly.Cite this article: Bone Joint Res 2022;11(8):575-584.

Variable bone mineral density reductions post-unicompartmental knee arthroplasty.

PURPOSE: Radiolucencies are commonly observed in unicompartmental knee arthroplasty (UKA) patients within 1 year of arthroplasty. The objective of the study was to identify how the bone mineral density (BMD) changes up to 1 year post-arthroplasty. METHODS: Dual X-ray absorptiometry scans were obtained from 11 UKA patients at 10 days and 3, 6, and 12 months post-surgery. Patients were scanned in both anteroposterior and lateral knee orientations. RESULTS: Most subjects saw a large decline in BMD in the first 6 months following surgery, followed by some recovery in bone mass. The biggest change occurred under the tibial intercondylar eminence, which decreased significantly by an average of 18 % at 6 months and was 15 % at 1 year. The average bone loss under the tibial tray was low; however, the bone loss at the anterior portion was higher with a significant average decrease of 14 %. There was no change in BMD under the tibial keel. There was significant bone loss of 13 % under the femoral component; the regions anterior and posterior to the central femoral implant peg both had significant bone loss of 14 %. The bone response between patients was very variable, with some patients losing bone steadily, and others gaining it rapidly after an early fall. CONCLUSIONS: While the overall reduction in BMD under both components was low, it was significant and there was substantial individual variation superimposed on this. Improving our understanding of this response to surgery may impact on prosthesis survival. LEVEL OF EVIDENCE: Therapeutic study: case series with no comparison group, Level IV.

Prediction of structural failure of tibial bone models under physiological loads: effect of CT density-modulus relationships.

Although finite element (FE) models can provide distinct benefits in understanding knee biomechanics, in particular the response of the knee to implants, their usefulness is limited by the modelling assumptions and input parameters. This study highlights the uncertainty of material input parameters derived from the literature and its limitation on the accuracy and usefulness of FE models of the tibia. An FE model of the intact human knee and a database of knee forces (muscles, ligaments and medial and lateral tibio-femoral contacts) were developed for walking and stair-descent activities. Ten models were constructed from ten different combinations of apparent bone density to elastic modulus material property relationships, published in the literature. Some of the published material property relationships led to predictions of bone strains in the proximal tibia which exceeded published failure criteria under loads imposed by normal activities. These relationships appear not to be applicable for the human tibia. There is a large discrepancy in proposed relationships that cover the cancellous bone density range. For FE models of the human tibia, the material relationship proposed by Morgan et al., which assumed species and anatomic site dependence, produced the most believable results for cancellous bone. In addition to casting doubt on the use of some of the published density-modulus relationships for analysis of the human proximal tibia, this study highlights the need for further experimental work to characterise the behaviour of bone with intermediate densities.

Validation of multiple subject-specific finite element models of unicompartmental knee replacement.

Accurate computer modelling of the fixation of unicompartmental knee replacements (UKRs) is a valuable design tool. However, models must be validated with in vitro mechanical tests to have confidence in the results. Ten fresh-frozen cadaveric knees with differing bone densities were CT-scanned to obtain geometry and bone density data, then implanted with cementless medial Oxford UKRs by an orthopaedic surgeon. Five strain gauge rosettes were attached to the tibia and femur of each knee and the bone constructs were mechanically tested. They were re-tested following implanting the cemented versions of the implants. Finite element models of four UKR tibiae and femora were developed. Sensitivity assessments and convergence studies were conducted to optimise modelling parameters. The cemented UKR pooled R(2) values for predicted versus measured bone strains were 0.85 and 0.92 for the tibia and femur respectively. The cementless UKR pooled R(2) values were slightly lower at 0.62 and 0.73 which may have been due to the irregularity of bone resections. The correlation of the results was attributed partly to the improved material property prediction method used in this project. This study is the first to validate multiple UKR tibiae and femora for bone strain across a range of specimen bone densities.