Introduction: Metal on metal hip replacement using large diameter bearings can be used as part of a hip resurfacing (HR) system or with a large diameter head total hip arthroplasty (LDH-THA). Both types of implant release metal ion, but the amount of ion released after LDH-THA has not been studied. The aim of the present study was to assess whole blood metal ion release at one year following LDH-THA.

Material and Method: Pre and post operative Cr, Co and Ti concentrations in whole blood were measured using a high resolution mass spectrometer (HR-ICP-MS) in 29 patients with LDH-THA (Durom LDH, Zimmer). The results were compared to published ion levels on a HR system (Durom, Zimmer) possessing the same tribological characteristics, the only differences being the presence of a modular sleeve and opened femoral head design in LDH-THA.

Summary of results: Post operative Cr, Co and Ti mean levels of LDH-THA were 1.3, 2.0 and 2.8 μg/L at 6 months and 1.3, 2.2 and 2.7 μg/L at 12 months. In the LDH-THA, the opened femoral head design showed significantly higher Co ion concentrations than the closed femoral head design (3.0 vs 1.8 ug/L, p=0.037). Compared to previously published results after HR, Co levels were significantly higher at one year in the LDH THA (2.2 ug/L vs. 0.7 ug/L, p< 0.001).

Discussion: In order to reduce wear and ion release from metal-metal bearing, most manufacturers focus research on improvements at the bearing surfaces. This study has demonstrated that the addition of a sleeve with modular junctions and an open femoral head design of LDH-THA causes more Co release than bearing surface wear (157% and 67% respectively). Even if no pathological metal ion threshold level has been determined, efforts should be made to minimize their release. We recommend modification or abandonment of the modular junction and femoral head closed design for this specific LDH-THA system. The total amount of ion released from a metal-metal implant should be considered globally and newer implant design should be scientifically evaluated before their widespread clinical use.

To compare the volume of acetabular bone resection after primary hip arthroplasty with different cup designs and technique of implantation using a computer model.

The factors influencing acetabular bone resection during acetabular cup implantation in THA or hip resurfacing (SRA) include the design of the component and technique of implantation. The impact of these variables on bone resection was simulated with a computer model. A 3-D pelvis was reconstructed from CT scan images. The bony acetabulum circumference was 52.5mm. Implantation of pressfit acetabular component sustaining angles of 165°, 170° and 180° with different wall thicknesses (3.5, 4.0, 5.0mm) at various depths was simulated.

Bone loss of 2742mm3 was calculated for the 165°, 4mm thick, 54mm cup, and deepening of reaming by 1 and 2mm would result in bone loss of 3780mm3 (+38%) and 5076mm3 (+85%), respectively. When oversizing to a 56mm 165° component, 4998mm3 (+82%) of bone was removed. For a 54mm, 5 mm thick component sustaining an angle of 180°, the bone loss would reach 12 410mm3 (+450%).

Acetabular component design has a significant influence on the amount of acetabular bone resection. The surgical technique (avoiding over deepening and oversised components) should minimise bone loss. This knowledge is of particular importance in hip resurfacing since the acetabular component size depends on the selected femoral component size. The knowledge is is also important in THA to minimise bone loss at primary implantation.

The purpose of this study was to compare the post operative ROM of patients randomised between SRA and 28mm THA.

Restoration of normal ROM has been proposed as an advantage of hip resurfacing (SRA) over THA and is due to the use of larger diameter femoral heads. However, the head-neck diameter ratio, which is an important factor governing ROM, would in theory allow more ROM with THA (28mm head/14mm neck = ratio 2:1) versus SRA (approximate ratio 1.3–2.0:1).

Patients were randomised between SRA and THA. Osseous landmarks were identified with a marker pen. Both ASIS served as the reference line for the pelvis position. Digital photographs of hip motion were taken and a blinded rater (with respect to the side and type of surgery) performed range of motion testing on the operated and normal side. Pre-study validation of ROM measurement method with a software program revealed high intra and inter observer reliability.

Sixty SRA and sixty-two THA were evaluated at minimum follow-up of twelve months. Preoperative ROM and demographic data were similar for both groups. No significant differences (p> 0.05) were found in the total arc of motion (SRA=204.2°, THA=196.5°), arc of rotation (SRA=47.7°, THA=44.3°), flexion-extension arc (SRA=118.1, THA=120.1), abduction-adduction arc (SRA=43.1°, THA=42.9°).

In theory, ROM should have been greater in THA. Fear of instability may have limited ROM recovery potential in THA. Since pre operative soft tissue contracture is an important factor influencing post operative ROM, the complete capsular release performed during SRA may have been an advantage of this technique.