Background

Dislocation is a major complication following total hip arthroplasty (THA). Aetiology is multi-factorial, but increasing femoral head size may result in lower dislocation rates. The latest England and Wales National Joint Registry (NJR) annual report has highlighted a statistically significant increase in the use of femoral heads of size 36mm+ from 5% in 2005 to 26% in 2009, together with an increase in the use of the posterior approach. The aim of this study was to determine whether national dislocation rates have fallen over the same period.

Metal-on-metal hip resurfacing prostheses are a relatively recent intervention for relieving the symptoms of common musculoskeletal diseases such as osteoarthritis. While some short term clinical studies have offered positive results, in a minority of cases there is a recognised issue of femoral fracture, which commonly occurs in the first few months following the operation. This problem has been explained by a surgeon's learning curve and notching of the femur but, to date, studies of explanted early fracture components have been limited.

Tribological analysis was carried out on fourteen retrieved femoral components of which twelve were revised after femoral fracture and two for avascular necrosis (AVN). Eight samples were Durom (Zimmer, Indiana, USA) devices and six were Articular Surface Replacements (ASR, DePuy, Leeds, United Kingdom). One AVN retrieval was a Durom, the other an ASR. The mean time to fracture was 3.4 months. The AVNs were retrieved after 16 months (Durom) and 38 months (ASR).

Volumetric wear rates were determined using a Mitutoyo Legex 322 co-ordinate measuring machine (scanning accuracy within 1 micron) and a bespoke computer program. The method was validated against gravimetric calculations for volumetric wear using a sample femoral head that was artificially worn in vitro. At 5mm³, 10mm³, and 15mm³ of material removal, the method was accurate to within 0.5mm³. Surface roughness data was collected using a Zygo NewView500 interferometer (resolution 1nm).

Mean wear rates of 17.74mm³/year were measured from the fracture components. Wear rates for the AVN retrievals were 0.43mm³/year and 3.45mm³/year. Mean roughness values of the fracture retrievals (PV = 0.754, RMS = 0.027) were similar to the AVNs (PV = 0.621, RMS = 0.030), though the AVNs had been in vivo for significantly longer.

Theoretical lubrication calculations were carried out which found that in both AVN retrievals and in seven of the twelve cases of femoral fracture the roughening was sufficient to change the lubrication regime from fluid film to mixed. Three of these surfaces were bordering on the boundary lubrication regime. The results show that even before the femoral fracture, wear rates and roughness values were high and the implants were performing poorly.

This case series highlights the use of the Ganz approach (trochanteric slide approach) and surgical dislocation for excision of fibrous dysplasia of the femoral neck, pigmented villonodular synovitis and synovial chondromatosis of the hip.

The first patient was a 16-year-old girl, who presented with pain in her hip, having fallen whilst playing football. Investigations revealed a fibrous dysplasia, which was successfully excised returning her to an active lifestyle.

The second patient was a 27-year-old lady, who presented having suffered left hip pain for four years. She was diagnosed with a pigmented villonodular synovitis, which was excised and the patient was able to return to the gym.

The third patient was a 41-year-old lady, who presented after experiencing right hip pain both at night and at rest for a year, without any trauma. She was diagnosed with synovial chondromatosis and returned to all activities of daily living.

The Ganz approach allows safe dislocation of the hip joint without the risk of osteonecrosis of the femoral head. We demonstrate that it is possible to obtain excellent exposure of the femoral neck, head and acetabulum to surgically treat these three tumours of the hip. The surgeon can thus be reassured that complete excision of the tumour has occurred.

This series can recommend the Ganz approach with trochanteric slide and full surgical dislocation of the hip to excise pigmented villonodular synovitis, synovial chondromatosis and fibrous dysplasia of the hip.