The results presented in section 6.2 have shown that femoral neck modularity influences both the size and position of the prosthetic range of motion area. This section aims to provide results which assess whether this influence is being effectively used to answer the second research sub-objective.
To assess if femoral neck modularity is being effectively used to relieve impingement. In chapter 3, section 3.4.4 presented two outcome measures. The first was defined as the propensity to impinge. This measured whether the prosthetic motion area was large enough and well positioned enough to encompass the graphical range of motion benchmark, shown in Figure 3.10 on page 64. If so, then it could be inferred that the post-operative outcome for that particular patient would be impingement free. This outcome measure was used to assess if the intervention treatment group with the benefit of femoral neck modularity provided a significant difference in the propensity to impinge in comparison to the control treatment group.
The second outcome measure was defined as impingement severity. This measured the degree of impingement if the prosthetic range of motion area was not large enough or well positioned enough to encompass the graphical range of motion benchmark. This outcome measure assessed, if impingement did occur, whether the intervention group provided a significant difference in the severity of impingement in comparison to the control treatment group. Finally, the outcome measure of propensity to impinge was also used to assess whether the ideal neck choice was made by the surgeon in the intervention treatment group. These three measures were used to assess whether femoral neck modularity was being effectively used in the clinical setting. This section presents the results of this experiment.
6.3.1 Clinical trial results
In chapter 3, an estimated sample size for each treatment group was determined. To accommodate for losses during the study, 61 patients were recruited for each treatment group. There were a total of 25 loses during the clinical trial representing a figure of 20.5%. Reasons for these loses included navigation failure, lack of navigation availability, a patient wishing to withdraw from the study and conversion from uncemented to cemented implants. Therefore, the control treatment group had 49 patients with 48 patients in the intervention treatment group. The results of the clinical trial, using the actual components that were implanted into the patients, shows that there was no significant difference in the propensity to impinge between the control and intervention groups, Table 6.9.
In chapter 3, impingement severity was defined as the size of the impingement area as a percentage of the graphical range of motion benchmark. Preceding the analysis, a Shapiro-Wilk test was conducted on the distribution of impingement area for the two cohorts and the results showed evidence of non-normality (Rees, 2001). Therefore, the canonical transformation of the impingement area was taken, which was its square root. The Shapiro-Wilk test, in this form, showed no evidence of non-normality in its distribution. Consequently, assessment of difference in the variance and mean of the two cohort distributions was done using this canonical transformation. The results of those patients who had evidence of impingement shows a significant difference in the mean impingement area between the two cohorts with the intervention group having a lesser degree of impingement as compared to the control treatment group, Table 6.10.
Control Intervention Total
Impinged 19 22 41
No Impingement 30 26 56
Total 49 48 97
p-value = 0.310
Control Intervention
Mean impinged area 9.3% 4.3%
Standard deviation 6.7% 3.7%
F-Test p-value (1-tail) 0.069 t-Test p-value (2-tail) 0.020
Table 6.10: Clinical trial result - impingement severity.
It was noted that differences in the other independent variables could contribute to a sig- nificant difference between the control and intervention groups with regard to impingement severity. It was shown in Table 6.1, that there were no significant differences between the acetabular cup inclination, anteversion and femoral stem version values of the two treatment groups. To assess the significance of having a modular femoral neck, all the modular necks in the intervention group were swapped post-operatively in the computer simulation with straight necks. Tables 6.11 and 6.12 analyses impingement propensity and severity between the control group and the non-modular intervention group. The results show that there would have been four more cases of impingement if a straight neck was consistently used in the intervention treatment group. Also, the severity of impingement between the control and the non-modular intervention is not significant, although there is a significant difference between their variances due to the greater amount of impinged cases in the non-modular intervention from which to calculate the F-statistic.
In the intervention group, to assess whether the most ideal modular femoral neck had been chosen in each case. Those 22 cases which had evidence of impingement were reprocessed post-operatively with alternative neck choices. Table 6.13, compares the actual choice of modular neck with the most ideal choice. Table 6.14, provides the propensity to impinge statistic between the control treatment group and the idealised modular neck intervention group. The results show that if the ideal neck was chosen in those cases that impinged, then this would have created a significant difference in the propensity to impinge when compared to the control group.
Control Intervention Total
Impinged 19 26 45
No Impingement 30 22 52
49 48 97
p-value = 0.094
Table 6.11: Non-modular intervention result - propensity to impinge.
Control Intervention
Mean impinged area 9.3% 6.0%
Standard deviation 6.7% 4.0%
F-Test p-value (1-tail) 0.038
t-Test p-value (2-tail) 0.202
Table 6.12: Non-modular intervention result - impingement severity.
Ante (15o) Ante (8o) Retro (15o) Retro (8o) Retro & Varus Retro & Valgus Neutral Actual 5 4 0 1 0 0 8 Ideal 8 2 1 0 0 0 1 Varus (15o) Varus (8o) Valgus (15o) Valgus (8o) Ante & Varus Ante & Valgus Actual 0 0 0 0 3 1 Ideal 6 4 0 0 0 0
Table 6.13: Intervention treatment group - modular neck choice.
Control Intervention Total
Impinged 19 9 28
No Impingement 30 39 69
49 48 97
p-value = 0.025