Marketing mix

There have been a number of clinical (Duwelius et al., 2010; Sakai et al., 2002; Toni et al., 2001; Traina et al., 2004, 2009a, b, 2011) and engineering (Miki et al., 2009; Sakai et al., 2000) evaluations of modular femoral necks. Toni et al. (2001) evaluated 347 cementless arthroplasties mainly treating primary osteoarthritis or DDH. The study found 100% cup and stem stability and patients reported a considerable improvement in pain, walking and range of motion one year post-operation. In terms of neck options selected, a straight neck was used 71.7% of the time. The study of Traina et al. (2004) and Duwelius et al. (2010) diagnosed their clinical cohorts into groupings based on assessment of their anatomy. Traina et al. (2004), found that modularity became more effective with increasing case difficulty, where 56.5% of patients classified as having a normal hip morphology received a straight neck, compared to only 21.6% of patients classified as having a severe hip deformity. In the Duwelius et al. (2010) study, over half the patients were classified as outlier patients meaning a non-standard modular neck was fitted to correct hip joint anatomy. Following treatment, both the Traina et al. (2004) and Duwelius et al. (2010) studies had a post- operative dislocation rate of 0.3%, better than the rate reported in the literature. Both studies noted how modularity corrected abnormal anatomy. Traina et al. (2004) also noted that the risk of impingement was reduced by the surgeon intra-operatively adjusting the femoral neck until optimum range of motion was achieved.

Sakai et al. (2002) employed CT in the assessment of 116 modular femoral neck cementless prostheses, in comparison to 23 non-modular cemented hips. It was found that the use of the retroverted neck in DDH patients resulted in post-operative version being significantly smaller in the modular femoral neck group. Also, the anteverted neck was able to increase post-operative anteversion in those patients who had a pre-operative version of less than 15o. It concluded that the modular neck system could be used to correct femora with various pre- operative conditions. However, there was a practical limit to the extent that the modular neck system could aid post-operative outcome to bring post-operative reconstruction within ideal limits.

(a)Fixed neck component. (b)Modular neck component.

Figure 2.14: Example inde-

pendent offset adjustment with modular necks (Traina et al., 2009a).

The studies by Traina et al. (2009a; 2009b; 2011) looked at the effectiveness of femoral neck modularity at providing benefit in a number of clinical scenarios. Traina et al. (2009a), analysed the effectiveness of modularity in restoring hip anatomy in both men and women. The study found a different distribution of neck choice between men and women, with women having shorter and more anteverted or retroverted necks fitted. It was stated that these differences correspond to those which are seen anatomically between men and women. The follow-up results showed that the dislocation ratio of women compared to men was 2:1, lower than the 4:1 ratio reported in literature. This was cited to be due to the flexibility of modularity in being able to restore hip anatomy in women as well as men. Traina et al. (2009b) assessed the effectiveness of implanted modular stems with a high centre of rotation, to enable an easier surgical technique in DDH patients. It comprised of 44 hips constructed with a normal anatomical centre of rotation and 44 hips with a high anatomical centre of rotation. The study found that there were no statistically significant differences in the radiographic and clinical results of both groups. This differed in comparison with other studies that had to construct a high hip centre through means other than femoral neck modularity. These cases usually suffered from poor clinical results with significant risk of aseptic loosening and prosthetic failure. Therefore, the modular femoral neck was found to be useful and effective in THA procedures where the acetabular cup cannot be placed in its

anatomical position because of poor bone stock. Finally, the study by Traina et al. (2011) analysed the 10 year follow-up results of 61 THAs diagnosed with secondary osteoarthritis due to DDH and fitted with a modular femoral neck. The clinical results of this series were good with hip function being restored almost every time with only one failure due to ceramic acetabular liner fracture.

The engineering investigations of Miki et al. (2009) and Sakai et al. (2000) used computer simulation and CT analysis of a medical model, respectively. Sakai et al. (2000) found that femoral modularity was useful in achieving the correct version and offset to prevent prosthetic, bony and soft-tissue impingement. However, similar to the author’s later clinical study they noted that there was a practical limit when trying to correct offset and version to within the normal 15o-30o range for severe cases of pre-operative anteversion. Therefore, the significance of this practical limit could be magnified if the acetabular cup is incorrectly positioned. Miki et al. (2009) found that modularity made range of motion until impingement less sensitive to acetabular cup position in comparison to a straight neck prosthesis. Therefore, as well being able to cope with a range of femoral morphologies, the surgeon can also adjust femoral version to suit a range of acetabular positions.

This section has presented a summary of the case series and engineering investigations that have assessed the effectiveness of femoral neck modularity. At present, the studies of femoral neck modularity have predominantly been cohort studies. This type of study inherently has a number of biases which can potentially affect the validity of the result (Chung and Burns, 2008; McCulloch et al., 2002). No studies have been found which have attempted to assess the potential benefits of femoral neck modularity with a suitable control - an equivalent hip implant without a modular femoral neck. This type of study, as assessed using a randomised controlled trial represents the highest level of evidence in clinical studies. As well as the case series and engineering investigations detailed in this section, there have been further engineering investigations and case reports relating to the integrity of the modular femoral neck and these are presented in section 2.6.3.