Abstract
Background
While not common in the native hip, occurrences of femoral head separation from the acetabular cup during gait are well documented after total hip arthroplasty. Although the effects of this phenomenon are not well understood, we hypothesize that these atypical kinematics are due to component misalignments that yield uncharacteristic forces on the hip joint that are not present in the native hip.
Objective
The objective of this study was to theoretically predict the causes of hip separation during stance phase using forward solution mathematical modelling.
Methods
The model simulates the quadriceps muscles, hamstring muscles, gluteus muscles, iliopsoas group, tensor fasciae latae, and an adductor muscle group. Other soft tissues include the patellar ligament and the ischiofemoral, iliofemoral, and pubofemoral hip capsular ligaments. The model was previously validated using telemetric implants and fluoroscopic results from existing implant designs.
The model is currently being used to analyze the effects that various surgical alignments have on hip separation. Specifically, this study analyzed 4 different hypothetical patients under the same 87 alignment conditions during stance phase. Alignment conditions include anatomical component alignment, intended acetabular cup medial and superior shifts, unintended cup medial and superior reaming errors, variations in cup version angles, leg length discrepancies, and femoral component offset modifications.
Results
During stance phase, it was determined that acetabular cup placement had a much more substantial effect on hip separation than femoral component placement. While neither femoral offset nor leg length discrepancy showed a correlation to hip separation, both medial and superior shifting of the acetabular cup showed a positive trend with increased hip separation. Figure 1 shows a comparison of average hip separation with intended shifts in the cup (0mm, 2mm, or 5mm) plus unintended reaming errors (0mm to 10mm extra) Furthermore, larger intended shifts in cup placement yielded smaller margins of error (Figure 2). Observe in Figure 2 how an increase in the size of the blue region (intended shifting region) correlates to a decrease in size of the green region (allowable error region where hip separation will not occur). It was also determined that cup version angles have less of a defined effect on hip separation, as the relationship between angular position and hip separation varied between patients.
Discussion
From our current analysis, the importance of proper alignment of the acetabular cup can be clearly seen. Overall, it has been shown that reaming errors of as low as 2 mm can yield separation magnitudes up to 2 mm (and potentially greater).
