VOL. 77-B, No. 6, NOVEMBER 1995 865 EFFECT OF FEMORAL OFFSET ON RANGE OF MOTION AND ABDUCTOR MUSCLE STRENGTH AFTER TOTAL HIP ARTHROPLASTY BRIAN J. McGRORY, BERNARD F. MORREY, THOMAS D. CAHALAN, KAI-NAN AN, MIGUEL E. CABANELA From the Mayo Clinic and Mayo Foundation, Rochester USA At a minimum of one year after operation, we studied 64 patients with 86 total hip arthroplasties (THA) by standard anteroposterior hip and pelvic radiographs and measurement of range of motion and of isometric abduction strength. The femoral offset correlated positively with the range of abduction (p = 0.046). Abduction strength correlated positively with both femoral offset (p = 0.0001) and the length of the abductor lever arm (p = 0.005). Using multiple regression, abduction strength correlated with height (p = 0.017), gender (p = 0.0005), range of fiexion (p = 0.047) and the abductor lever arm (p = 0.060). Our findings suggest that greater femoral offset after THA allows both an increased range of abduction and greater abductor strength. J Bone Joint Surg [Br] 1995;77-B:865-9. Received 22 June 1994; ; Accepted after revision 23 March 1995 There has been recent interest in femoral offset as a factor in total hip arthroplasty (THA) (Yanagimoto 1991; Abra- ham and Dimon 1992; Steinberg and Harris 1992; Davey et al 1993; Rothman et al 1993; Wong et al 1993). Femoral offset is defined as the perpendicular distance between the long axis of the femur and the centre of rotation of the femoral head. Charnley (1979) considered it to be a factor under the control of the surgeon at the time of hip replace- ment surgery; the more lateral position of the femur with greater offset was said to increase the range of motion and B. J. McGrory, MD, Senior Resident Orthopaedic Associates of Portland, P0 Box 1260, 33 Sewall Street, Portland, Maine 04104-1260, USA. B. F. Morrey, MD, Chairman, Department of Orthopaedics T. D. Cahalan, RN’, Physical Therapist K-N. An, PhD, Director, Biomechanics Laboratory M. E. Cabanela, MD, Professor of Orthopaedics Mayo Clinic and Mayo Foundation, 200 First Street SW, Rochester, Minnesota 35905, USA. Correspondence should be sent to Dr B. F. Morrey. 01995 British Editorial Society of Bone and Joint Surgery 0301-620X/95/61066 $2.00 decrease the incidence of impingement of the femur on the pelvis. An increase in femoral offset (and consequently of the lever arm of the abductor muscles) will also, theoret- ically, increase the mechanical advantage and strength of the abductors. Finally, a greater femoral offset will increase stability by preventing impingement and improving soft- tissue tension. Femoral offset has been shown to correlate with hip stability (Fackler and Poss 1980; Huk et al 1993), but we know of no clinical study which has related the range of motion or abduction strength to femoral offset after THA. We therefore reviewed 64 consecutive patients at a mini- mum of one year after surgery in an attempt to answer these questions. PATIENTS AND METHODS We reviewed 64 patients who had a total of 86 THAs by one of two surgeons (BFM, MEC) at a minimum of one year (mean one year nine months, range one year to three years two months) after surgery, by radiography, examina- tion, and standard abductor strength testing. The number of joint replacements included was based on a power analysis calculation (see statistical section). The minimum follow- up was chosen because results for THA have been shown to stabilise at one year (Insall et al 1983; Schurman, Parker and Ornstein 1985; Parsley, Engh and Dwyer 1992). We excluded patients with a diagnosis of rheumatoid arthritis to avoid confounding factors in strength measurements. Patients who would have had to travel more than 500 miles were also excluded; the one-year assessment for such patients is often carried out locally. The patients had been operated on over a two-year period, from 1988 to 1989, during which 263 THAs (excluding patients with a diagnosis of rheumatoid arthritis) had been performed by the two surgeons. Informed consent was obtained and the project and consent process were approved by the IRB of the Mayo Clinic. There were 36 women and 28 men and the median age was 60 years (22 to 87). Their average weight and height were 77.7 kg (44.5 to 109.1) and 168 cm (140 to 190). All were seen in follow-up from April 1, 1989 to April 1, 1993 and no evaluations were excluded. The right hip had been replaced in 48 (55.8%) and the left in 38. The diagnosis was osteoarthritis in 57 (66.3%), post-traumatic deformity