Patients with developmental dysplasia of hip often undergo total hip arthroplasty (THA) at a younger age. THA in such patients are technically demanding because of deficient acetabular bone stock, abnormal femoral anatomy with increased anteversion, leg length discrepancy and soft tissue imbalance. Conical femoral stem with Metal on Metal (MoM) 28 mm bearing have been used to address the anatomical and wear issues in this young patient group. The aim of the study was to review the early midterm clinical results of this prosthesis.
Material & Methods
Between January 2004 and March 2009 twenty-one consecutive patients (24 hips) with osteoarthritis secondary to developmental dysplasia of the hip underwent metal-on-metal uncemented Allofit Metasul press-fit cups (Zimmer) with 28 mm-heads and tapered uncemented Wagner cone prosthesis stems (Zimmer) using posterior approach.
Functional outcomes were assessed using Oxford hip score (OHS), SF12 physical and mental scores and WOMAC scores. Radiographic outcomes was carried out by two independent observers for leg length discrepancy (LLD), Crowe and Tonnis grading, lateral center -edge (CE) angle, acetabular roof-arc angle and femur neck-shaft angle heterotropic ossification, prosthesis loosening and cup inclination angle.
There were 5 men and 16 women with mean age of 45 years (SD=9.15, 26-63). The mean follow up was four years (SD=1.7, 2-7). No patients were lost to follow up. Three patients required structural allograft, no patients required intra operative femoral shortening. Classification of DDH was Crowe type I in 14 hips, type II in 8 hips and type III in 2 hips. The mean preoperative CE angle was 10.5°, neck shaft angle was 143° and the acetabular roof arc angle was 25°. 3 hips had Tonnis Grade 2 while 21 had Grade 3 osteoarthritis. The mean cup inclination angle was 42 degrees (range 32-60 degrees). The mean preoperative limb shortening was 19.1 mm (range,-1 to -39) which improved to 2.8 mm postoperatively.
The OHS improved significantly within subjects (p<0.001) from a mean of 10.6 (2-28) to 43.9 (30-48). The mean post operative scores were- WOMAC 51 (SD=7.4, 30-56), SF12 physical 51 (SD=8.5, 28-58) and SF12 mental 53 (SD=4.7, 46-59). There were two revisions for aseptic loosening, one stem and one cup; therefore giving an overall survival of 91%. No other hip had radiographic evidence of acetabular or femoral osteolysis. Three patients suffered transient femoral nerve palsy, which recovered completely. We did not observe any dislocation, deep infection or heterotopic ossification.
The medium-term results of the small diameter metal-on-metal hip arthroplasty with a conical uncemented stem have been encouraging in the treatment of this difficult cohort of young patients. The inherent stability and functional outcome provided by the small femoral heads are better than the recent large metal on metal articulations. Further follow up is required to see how small MoM bearing will behave in the long term.