The functional outcome after total hip arthroplasty (THA) depends on the status of the stabilizing periarticular muscles. Postoperative (pOP) muscle atrophy and fatty degeneration may alter muscle function and reduce mobility. An increase in fatty degeneration of gluteal muscles have been shown to correlate with impaired gait patterns such as Trendelenburg limb, which consecutively can lead to higher joint loading in the hip (1, 2) (see videos).
The aim of this study is to investigate the influence of the hip muscle status on the in vivo measured hip joint contact forces during activities of daily life (ADL) in THA patients.
Ten patients (n=10), who underwent THA with an instrumented implant (3) were examined at 3 months and 50 months pOP. All patients were operated using a direct lateral approach. Follow up included individual CT scans of the pelvis and lower limbs and measurements of in vivo hip joint loads during ADLs (walking, stair climbing, sitting down/standing up from a chair).
Muscle volumes of the ipsilateral gluteal muscles and tensor fascia latae muscle (TFL) were calculated using Amira Imaging software (Visage Imaging, Berlin, Germany).
The degree of fatty degeneration was calculated in Image J 1.44 (Bethesda, Maryland, USA) using standardized methods (4).
We focused our analysis on the load cycle moments of the gluteus minimus muscle (Gmin) and TFL (Fig. 1) as these muscles have been shown to be active in these movement phases (5).
Statistical analyses were performed using the Pearson correlation test. A p-value of 0.05 was considered significant.
The mean muscle volume change of the Gmin was -24.5 (SD 15.3)% at 3 months pOP, resp. -18.45 (SD 10.8)% at 50 months pOP. The volume change of the TFL was on average +14.4 (SD 21.9)% at 3 months and +14.4 (21.9)% at 50 months.
The degree of fatty degeneration of the Gmin (Fig. 3, left) increased at 3 months by +30.9 (SD 56.1)% and by +89.8 (SD 166.6)% at 50 months whereas it decreased in the TFL (Fig. 3, right) by -56.7 (SD 32.7)% at 3 months and by -32.2 (SD 50.8)% at 50 months.
Correlations were found to be significant for the Gmin fat ratio and the hip load during sitting down and standing up (R=0.928, p=0.01; R=0,841, p=0.04). The muscle volumes didn’t show significant correlations.
We found the ipsilateral Gmin to increase in fat ratio 3 and 50 months after surgery and the TFL to decrease at both time points (Fig. 2). This was accompanied with an inverse development of volume. This mechanism has also been shown in previous studies and could be validated with our data (5-7). Our preliminary data analyses indicates that higher fat ratios of Gmin resp. TFL lead to an increase of the hip joint contact force at 3 months pOP. At 50 months pOP such relationship between fat ratio and contact force of the Gmin muscle could not be shown anymore, but was still visible for the TFL. This could indicate that the TFL compensates a decreased function of the Gmin.
Overall the fat ratio (quality) of the muscle seems to be a better indicator of resulting joint loads than muscle volume (quantity). Further evaluation of our patient data including lever arms of individual hip muscles, electromyography (EMG) and clinical scores will be necessary to fully understand the relation between the muscle status and in vivo hip joint contact forces.
(1) Bergmann et al., 2001; (2) von Roth et al., 2014 (3) Damm et al., 2010; (4) Daguet et al., 2011; (5) Correa et al., 2010; (6) Rodríguez-Roiz et al., 2016; (7) Springer et al., 2012; (8) Sutter et al., 2013