Mismatch between trochlear coronal alignment of arthritic knees and currently available prosthesis: a morphological analysis of 4116 knees and 45 implant designs

Abstract

Purpose In up to a fifth of total knee replacements (TKR), surgeons are not capable of achieving good clinical and functional results. Despite comprehensive diagnostic workup, an underlying cause is not always identified in these patients. The purpose of this study is to compare native and prosthetic trochlear anatomies, to evaluate a potential source of morphologic mismatch and theoretically, of poor clinical outcomes. Methods Native trochlear angles of 4116 knee CTs from 360 Knee Systems database of arthritic pre-operative TKR patients were evaluated. A semi-automated tridimensional analysis was performed to define the native trochlear angle in the coronal plane (NTA) among other 142 parameters. An active search was conducted to identify currently available TKR models; prosthetic trochlear orientation in the coronal plane (PTA) was extracted from the technical data provided by manufacturers. Results The mean native trochlear angle (NTA) was 1.6° ± 6.6° (valgus) with a range from − 23.8° (varus) to 30.3°(valgus). A valgus NTA was present in 60.6% of the knees and 39.4% of them had a varus NTA. 89 TKR models were identified; trochlear details were available for 45 of them, of which 93% were designed with a valgus orientation of the prosthetic trochlear angle (PTA) and 6.9% showed a neutral (0°) PTA. Varus alignment of PTA was not present in any system. Angular numeric values for PTA were available for 34 models; these ranged from 0° to 15° of valgus, with a median value of 6.18° (SD ± 2.88°). Conclusion This study shows a significant mismatch between native and prosthetic trochlear angles. A relevant proportion of the studied knees (41.45%) fall out of the trochlear angle range of currently available implants; representing a potential source for biomechanical imbalance. While further research is warranted to fully understand the clinical implications of the present study, manufacturers may need to take these findings into account for future implant designs. Level of evidence Level III, retrospective cohort study.

Better forgotten joint scores when the angle of the prosthetic trochlea is lateral to the quadriceps vector in kinematically aligned total knee arthroplasty

“…The 2D radiographic findings of the present study, which showed a femoral component with a PTA of 6° valgus did not include the Q-angle in MA TKA simulations, is consistent with 3-D analysis studies that showed that MA TKA does not restore native trochlear morphology [14,22,24]. For example, a study of 45 femoral component designs aligned mechanically in 4116 3-D CT femoral models showed that 41% of the PTA fell out of the range of the native trochlear angle, representing a potential source for biomechanical imbalance, which manufacturers may need to take into account for future implant designs [24]. Another 3-D analysis showed the optimal target for aligning the femoral component is not the FMA because the QV passes lateral to the femoral head center [16].…”

“…However, as presented in Fig. 1, most of the femoral components from the major implant manufacturers have a PTA of 6° valgus, similar to the one evaluated in the present study [24]. Finally, the present study did not evaluate the clinical consequences of medial and lateral deviation for MA TKA, so it remains unknown whether the adverse clinical consequences of medial deviation of the PTA reported for unKA also apply to MA TKA without further study.…”

“…The most important findings were that a PTA of 6° valgus, which is the mean orientation of most femoral components, deviated medially relative to the QV or patient’s Q‐angle in 86% of patients after unKA TKA, which caused a 17‐point worse FJS relative to those with lateral deviation at a mean follow‐up of 4 years, which exceeded the reported 14‐point MCID value by 3 points [4, 24]. A secondary finding was that medial deviation occurred after MA TKA, though the frequency was significantly less than unKA TKA.…”

“…Previous reports of 3‐D analysis of the difference between the femoral component positioned with unKA and the native trochlea have generally confirmed the present study's findings that a femoral component with a PTA of 6° valgus positioned with unKA does not restore the native trochlea in all patients [14, 15, 22–24]. One explanation, according to a CT analysis of 4116 knees, is that the angle formed by the native trochlear angle (i.e., sulcus) and distal femoral joint line varies widely from 24° varus to 30° valgus, which suggests that existing arthroplasty components and techniques may not replicate the anatomy of morphologic outliers and could lead to dissatisfaction [9, 21, 24, 26]. A second explanation is that four 3‐D analysis studies reported a difference between three femoral component designs positioned with unKA and the native trochlea and concluded that although randomized trials have not shown differences in patellofemoral complications between KA TKA and MA TKA, a femoral component explicitly designed for KA that more closely restores the native trochlear anatomy might improve patient‐reported satisfaction and function [14, 15, 21, 22].…”

“…In total knee arthroplasty (TKA), one key feature of any femoral component is the prosthetic trochlear angle (PTA), measured relative to a vertical line drawn perpendicular to the distal joint line. For most femoral components, the mean PTA is 6° lateral (range 0°–15°) because it coincides with the quadriceps vector (QV) when the femoral component is mechanically aligned (MA) perpendicular to the femoral mechanical axis (FMA) [2, 7, 8, 24, 27] (Fig. 1).…”