Abstract
BACKGROUND & AIM
Most previous studies found that the rate of dislocation following primary THA was 2 to 3 % on average. It is expected that minimally invasive (MI) THA has fewer dislocations after surgery because it causes less muscle damage. To ascertain the risk factors of dislocation, we conducted a retrospective study of the occurrence of dislocation after MI-THA in Japanese patients.
METHODS
From June 2003 when we began MI-THA to August 2010 primary MI-THAs were performed on 2,042 hips; 1,997 hips with mini-posterior approach (a mean incision of 7 cm) with the repair of posterior soft tissues and 45 hips with other approaches. The dislocation after MI-THA was studied with respect to age, sex, body mass index, the use of navigation system, femoral head size, cup size and approaches. The period of follow-up was from six weeks to seven years.
RESULTS
The numbers of dislocation after MI-THA were 13 hips (0.6%). The numbers of posterior and anterior dislocation after MI-THA were nine and four hips, respectively. In the patients undergoing MI-THA, there was a significant difference between non-dislocated hips and dislocated hips, with respect to the average age (57 years vs. 66 years, p<0.017), the numbers of male patients (181 cases (9%) vs. 4 cases (31%), p<0.024), cup size (50 mm vs. 53 mm, p<0.007) and the numbers of cases which used navigation system (1,932 hips (95%) vs. 10 hips (77%), p<0.023). Whereas there was no significant difference between non-dislocated hips and dislocated hips with respect to the body mass index, femoral head size and approaches.
DISCUSSION
Several risk factors of the dislocation after primary THA have been reported. In this study we found that MI-THA had fewer risk of dislocation as compared with historic controls. It is suggested that less soft tissue damage can decrease the risk of the dislocation after surgery. The incidence of dislocation was fewer in the younger and female patients undergoing MI-THA with the navigation system than in the older and male patients undergoing MI-THA without the navigation system. The posterior approach combined with the repair of posterior soft tissues did not increase the risk of dislocation after surgery as compared with other approaches. The position of implants is important to prevent dislocation after surgery, and the navigation system can help to obtain a good position of implants.
CONCLUSIONS
We conclude that MI-THA can decrease the risk of dislocation after primary THA. Furthermore the combination of MI-THA and the navigation system is very useful to reduce the incidence of dislocation because the use of the navigation system during surgery can be helpful to acquire the precise position of implants.
