Above knee socket prosthesis use changes proximal femur morphology

Abstract

Patients with transfemoral amputation (TFA) are up to six times more likely to develop hip osteoarthritis (OA) in either or both the intact and residual limb, which is primarily attributed to habitually altered joint loading due to compensatory movement patterns. However, joint loading patterns differ between limbs, which confounds the understanding of loading-induced OA etiology across limbs. It remains unknown if altered loading due to amputation results in bony shape changes at the hip, which is a known etiological factor in the development of hip OA. Retrospective computed tomography images were collected of the residual limb for 31 patients with unilateral TFA (13F/18M; age: 51.7 ± 9.9 y/o; time since amputation: 13.7 ± 12.4 years) and proximal femur for a control group of 29 patients (13F/16M; age: 42.0 ± 12.27 years) and used to create 3D geometries of the proximal femur. Femoral 3D geometric variation was quantified using statistical shape modeling (SSM), a computational tool which placed 2048 corresponding particles on each geometry. Independent modes of variation were created using principal component analysis. 2D radiographic measures of the proximal femur, including common measures such as α-angle, head neck offset, and neck shaft angle, were quantified on digitally reconstructed radiographs (DRRs). SSM results were then compared to 2D measures using Pearson correlation coefficients (r). Two-sample t-tests were used to determine if there were significant differences between the TFA and control group means of 2D radiographic measurements (p < 0.05). Patients with TFA had greater femoral head asphericity within the SSM, which was moderately correlated to head-neck offset (r = -0.54) and α-angle (r = 0.63), as well as greater trochanteric torsion, which was strongly correlated to the novel radiographic measure of trochanteric torsion (r = -0.78), compared to controls. For 2D measures, the neck-shaft angle was smaller in the TFA group compared to the control group (p = 0.01) while greater trochanter height was larger in the TFA group compared to the control group (p = 0.04). These results indicate altered loading from transfemoral prosthesis use changes proximal femur bony morphology, including femoral head asphericity and greater trochanter changes. Greater trochanter morphologic changes, though not a known factor to OA, affect moment arm and line of action of the primary hip abductors, the major muscles which contribute to joint loading and hip stability. Thus, chronic altered loading of the amputated limb hip, whether under- or overloading, results in bony changes to the proximal femur which may contribute to the etiological progression and development of OA.

Keywords: Above knee amputation; Digitally reconstructed radiographs; Femur morphology; Socket prosthesis; Statistical shape modeling; Transfemoral amputation.

Figure 1:. Statistical shape modeling (SSM) workflow.

CTs were used to create 3D proximal femur reconstructions and 2,048 correspondence particles are placed on each surface.

Figure 2:. Digitally reconstructed radiographic views and measurements.

Three radiographic views, a) anteroposterior, b) axial, and c) frog-leg lateral, were digitally reconstructed from CT and the 2D measures were made on corresponding views.

Figure 3:. Mean shape differences.

A) Mean proximal femur shape from the SSM for the TFA group (blue) imposed on the control group mean shape (red). B) The color map is given on the mean proximal femur shape of the control group and displays group-wise p-values for differences between the mean TFA and control shapes, with areas in blue indicating statistically different shapes (p < 0.05).

Figure 4:. Resultant cumulative variance for all SSMs.

The first nine modes account for 90.9% of the variance within the control group, 91.0% of the variance within the TFA group. The dotted line indicates 90% VAF, commonly reported for SSMs [28,30].

Figure 5:. Significant shape modes for the TFA and control group.

The color map femur displays distance from the shape mode to the mean shape for both control and TFA. Mode five shape variation is not included for the control group as it was spurious. Areas of darker red or blue indicate areas which have varied more from the means shape.