Aims: Patient-related risk factors of dislocation after total hip arthroplasty (THA) that have been identified are previous hip surgery, old age and female gender. However, there have been no prospective reports whether inflammatory arthritis (IA) is an independent risk factor. Methods: Prospective evaluation of the incidence of early (< 2 year post-surgery) dislocation in a consecutive series of primary THA. From January 1996 to December 1999 341 THAs in 311 patients with osteoarthrosis (OA) and 69 THAs in 59 patients with IA (mainly rheumatoid arthritis) were included in this study. One type of prosthesis having a 28 mm. ball head was implanted in every hip through an anterior appoach. Results: Both groups were comparable with respect to the following risk factors: gender, position of the acetabular component and experience of the surgeon. Average age was lower in the IA group than in the OA group: 61.0 vs 68.1 years. Furthermore, the incidence of previous hip surgery was higher in the OA group. Despite the presence of these risk factors in the OA group, the incidence of dislocation was higher in IA than in OA: 10.1% vs. 2.9% (p=0.006). All dislocations in IA where posterior, in OA 5 were posterior and 4 were anterior (1 unknown). No other mechanical factors leading to an increased instability of the hip in IA, such as trochanteric fractures, could be identified. Conclusions: Inflammatory arthritis is an independent risk factor of dislocation after THA. Both the polyarticular impairments and the lower quality of the soft tissues in IA could explain this increased risk.

Introduction

Dislocation after primary total hip arthroplasty (THA) is a devastating and frequent postoperative complication. Many risk factors for dislocation have been identified, however, thus far there has been no consensus whether inflammatory arthritis is a risk factor for dislocation or not. We carried out a prospective study assessing the prevalence of dislocation within 2 years after primary total hip arthroplasty for osteoarthrosis and inflammatory arthritis.

Patients and Methods

Between 1996 and 1999 312 patients (342 hips) with either a primary or a posttraumatic osteoarthrosis (OA group) and 59 patients (69 hips) with rheumatoid arthritis or other forms of inflammatory arthritis (IA group) were operated. One single type of prosthesis was implanted (EPF-PLUS^® cup and SL-PLUS^® stem) using an anterior approach. All dislocations in the two years following surgery were recorded. Both diagnostic groups were compared for known risk factors such as old age, female gender, prior hip surgery, and experience of the surgeon. Radiographs were examined for avulsion fractures of the tip of the trochanter and signs of loosening. The abduction and anteversion angles of the acetabular component were measured. Statistical analysis was performed with the Chi-square test and Student’s t-test.

Results

The dislocation rate for inflammatory arthritis patients was significantly greater than that in patients with osteoarthrosis: 10. 1% (7 hips) in the IA group, 2. 9% (10 hips) in the OA group (p = 0. 006). There were no other differences in risk factors favouring dislocation in the IA group, such as old age, female gender, prior hip surgery, experience of the surgeon, trochanteric fractures or malposition of the prosthetic components. All dislocations in the IA group were posterior and occurred without any kind of trauma. In contrast, nearly half of the dislocations in the OA group were anterior and two were of traumatic origin.

Discussion

Taking into account the fact that there are no differences in known risk factors for dislocation between our two groups and no differences in complication rate, except for dislocation, we can say that inflammatory arthritis has to be considered an independent risk-factor for dislocation after primary total hip arthroplasty. It may be that inferior quality of the (pseudo) capsule and the muscles stabilising the hip joint due to inflammatory arthritis leads to inadequate soft tissue tension. Another factor can be the concomitant impairments in rheumatoid patients, such as impairments of the upper extremity, ipsilateral knee or contralateral hip, leading to hyperflexion in the operated hip causing a posterior dislocation without trauma.