Muscle mass and function after total hip arthroplasty

Abstract: Osteoarthritis (OA) of the hip is a common disease among elderly causing pain, joint stiffness and reduced mobility. Outcome studies have shown total hip arthroplasy (THA) to be a successful surgical procedure. Studies of muscle strength and function after THA are more scarce and results vary. It has been suggested that unloading of the OA limb due to pain, results in hip and thigh muscle weakness and atrophy causing an abnormal gait and impaired postural control. Muscle atrophy can be quantified with computerized tomography (CT) by determination of cross-sectional area (CSA) and radiological density (RD; in Hounsfield units: HU). Atrophy will manifest as a reduced CSA and RD, where a lowered RD represents a muscular fatty infiltration. The aim of this thesis was to characterize muscle strength, atrophy, gait and postural stability in patients with unilateral hip OA before and after operation with THA. We hypothesized that muscles would not recover fully after operation. We have evaluated the reproducibility of a dynamometer assessing maximal isometric voluntary force of hip and knee muscles and an opto-sensor walkway detecting limp. A test-retest design was used. Ten young and thirteen aged healthy volunteers and eleven patients with unilateral hip OA were tested for muscular strength. Twenty-five volunteers underwent gait analysis. Coefficient of variation (CV%) for unilateral strength measurements ranged between 7-12 % and for gait parameters between 4-8 %. Twenty patients with unilateral OA were assessed preoperatively, 6 months and two years after THA for strength of hip and knee muscles, gait, postural control and clinical scores (HHS, SF-36, EQ-5D). Also, CSA and RD of hip, thigh, calf and back muscles were assessed using CT. Preoperatively, strength in OA relative to the healthy limb was reduced by 9-27 % in all muscles except knee flexors. CSA was reduced by 5-15 %, except for gluteus medius/minimus and ankle plantar flexors and RD was reduced by 3-14 HU. Gait analysis demonstrated a shorter single stance phase (limp) for the OA compared to the healthy limb preoperatively. No significant difference in postural control between healthy and OA limb could be demonstrated. At the two years follow-up, hip muscles showed a remaining 6 % weakness in OA compared to the healthy limb. Preoperatively and 6 months postoperatively that deficit was 18 % and 12 %, respectively. Among individual muscles the largest deficit (15%) was observed in hip abductors. Knee extensors and calf muscles recovered fully. There was still a reduction in CSA for m. iliopsoas (7.0 %) and hip adductors (8.4 %) and in RD for mm. gluteus maximus (10.1 HU), gluteus medius/minimus (5.6 HU), iliopsoas (3.9 HU) and adductors (2.4 HU). Limp was recovered already at the 6 month follow-up. Bilateral postural stability and all clinical scores improved after operation. We concluded that our dynamometer system and technique for gait analysis provides reliable measurements. Muscles acting about the hip and knee joints showed substantial loss in strength and mass before operation. Decreased muscle CSA could not fully explain the strength loss. Infiltration with fat in OA limb muscles was substantial and if not adjusted for there is a risk that muscle atrophy is underestimated. Two years after THA there is a persisting hip muscle atrophy and weakness, marked at 6 months. An earlier operation, a less invasive surgical trauma or a more qualified rehabilitation model, targeting hip abductors might speed up muscular recovery.