Restoration of leg length and offset is an important goal in total hip replacement. This paper reports a calliper-based technique to help achieve these goals by restoring the location of the centre of the femoral head. This was validated first by using a co-ordinate measuring machine to see how closely the calliper technique could record and restore the centre of the femoral head when simulating hip replacement on Sawbone femur, and secondly by using CT in patients undergoing hip replacement.

Results from the co-ordinate measuring machine showed that the centre of the femoral head was predicted by the calliper to within 4.3 mm for offset (mean 1.6 (95% confidence interval (CI) 0.4 to 2.8)) and 2.4 mm for vertical height (mean -0.6 (95% CI -1.4 to 0.2)). The CT scans showed that offset and vertical height were restored to within 8 mm (mean -1 (95% CI -2.1 to 0.6)) and -14 mm (mean 4 (95% CI 1.8 to 4.3)), respectively.

Accurate assessment and restoration of the centre of the femoral head is feasible with a calliper. It is quick, inexpensive, simple to use and can be applied to any design of femoral component.

Work carried out by Bennett [1], identified a link between patient gait pattern and total hip prothesis wear rate. This study found that the shape of the patient gait pattern (as quantified by aspect ratio) and sliding distance of the movement loci were found to have an improved positive correlation with wear rate compared to the factors of activity and patient weight. The distribution of theoretical shear stresses at selected points on the acetabular cup suggests that orientation of the polymer chains may occur. Wang et al, 1997 [2] has shown that failure of the UHMWPE wear surfaces occurs in the form of transverse rupture between oriented molecules.

This work investigates the hypothesis that the gait pattern of pre-revision THR patients has an effect on the wear, surface characteristics and material properties of the artificial hip joint, in particular the degradation of chemical and mechanical properties of the UHMWPE acetabular socket. Gait analysis is performed on patients prior to revision of a primary THR, with the retrieved socket used for subsequent analysis.

Chemical and mechanical analysis of a large number of retrieved UHMWPE acetabular sockets has shown clear structural changes, which are dependent on the length on time in-vivo. Increasing the length of time in-vivo between 2 and 20 years results in an increase in the percentage crystallinity of the UHMWPE of 12.7 %. A positive linear correlation (R2 = 0.765) between percentage crystallinity and number of years in-vivo is shown. This suggests recrystallisation of the polymer at a constant rate over time. This partial recrystallisation of the amorphous region correlates with degradation in the mechanical properties of the material. This pilot study aims to assess the effect of patient gait pattern on the chemical and mechanical degradation of UHMWPE, which will ultimately affect the clinical performance of the prothesis.

Recent studies, including the Swedish Hip Register, have confirmed that modern cementing techniques are important to achieve long-term implant survival. Their ultimate goal is to obtain an increased strength of cement and its interfaces with bone, thereby maintaining secure fixation and effective load transmission.

The objective of this study was to measure the medullary pressures generated during bone cement injection, pressurisation and femoral prosthesis insertion for total hip arthroplasty. The measurements were recorded throughout the length of an in vitro femoral model while implanting a series of prosthetic hip stems using different pressurisation techniques. The prostheses used were the Charnley 40 flanged stem, an Exeter No. 3 stem, and a custom primary femoral component used in Belfast (Johnson & Johnson, DePuy International Ltd.). The following parameters were derived from the pressure data recorded; peak pressure, decay pressure and duration above 76 kPa, the pressure regarded as the threshold to obtain adequate bone penetration.

The range of peak distal stem pressures expected for all stems was 200–500kPa. The custom and Exeter stems generated proximal cement pressures in the range 100–300kPa. These pressures were attained through cement containment by stem design or auxiliary pressurising devices, respectively. It was observed that the Charnley femoral component did not perform as well with regard to proximal pressurisation, irrespective of which pressurisation technique was implemented. The durations of pressure maintenance above 76kPa are also important, 5 seconds being accepted as a minimum for an effective interface. These results reflect the pressure measurements, with adequate durations being maintained by those stems and pressurisers that were characterised by higher peak pressures.

It is concluded that stem design and the complementary cement management techniques are essential to realise the pressure/time characteristics that are regarded as necessary to form an optimum bone/cement interface.