The present study was undertaken to investigate changes in bone remodeling in the upper part of the femur during the first postoperative year after BHR. Our results show that, although three months after operation the BMD levels were inferior in six out of ten ROIs on the operated side compared to preoperative level, the BMD recovered and even exceeded the preoperative level in seven ROIs at the one-year follow-up.
It is widely accepted that BMD of the proximal femur generally decreases after total hip replacement [8, 10, 14, 15]. The reason for this resorption is mainly due to stress-related remodeling during the early postoperative period. On the other hand, the use of the bone conserving HRA method is reported to preserve femoral BMD better than THA due to better load transfer to the proximal femur [16, 17]. In addition to implant-related factors, the decrease of BMD can partly be explained by patient related factors like decreased mobility. The present patients were allowed to bear full weight immediately after the operation, although a cane was recommended for 2 weeks afterwards to help with balance. In the study by Lian et al. (2008), partial weight bearing was allowed during the first two weeks, progressing then to full weight bearing over the next two months . As a decrease in bone stock of the proximal femur was found in both of these studies 3 months postoperatively, the small difference in postoperative weight-bearing mobility did not influence bone loss. According to self-reported physical activity diaries, the time spent doing physical activity per day was at the preoperative level at the 3-month follow-up in our patients. This activity level was sufficient to maintain the bone stock of the contra-lateral hip showing that the effect of weight-bearing activities on BMD was smaller than the effect of the surgical procedure.
Another explanation for early bone loss might be impaired vascularization due to HRA [18, 6]. Steffen et al. (2005) reported a 62% reduction in femoral head oxygenation during the operation; in some patients, the blood supply was completely lost postoperatively . During resurfacing through a posterior approach, the branches of the medial femoral circumflex artery, which pass along the short external rotators, are likely to be destroyed. This deep branch provides the most important blood supply to the femoral head. In these patients, the posterior approach was used and short rotators were cut. However, the femoral neck was left intact preserving the branches of the femoral circumflex arteries. This should ensure blood and nutrition supplies reach the proximal femur and, thus, prevent bone loss due to impaired vascularization . As the BMD of all ROIs reached at least the preoperative level or even exceeded it, we may assume that blood supply had recovered sufficiently in the present patient sample to support bone remodeling during the one-year follow-up.
In our study, significant new bone stock formation appeared during the one-year follow-up. Thus, hip resurfacing seems to transfer the load to the upper femur, as occurs physiologically. The operation was extremely effective in reducing hip pain during the loading, promoting the patients' walking capacities and physical activity levels. Thus, an increase in weight bearing activities may have, in part, also improved the mechanical loading of the hip. However, the surgical procedure, particularly the division of the external hip rotator muscles, led to specific deficit of external rotation strength and the active range of movements as we have reported earlier . Another recent study also reported HRA patients have significantly decreased hip extension and flexion angles and asymmetric gaits 18 months after surgery . These deficits may increase the risk of dislocations, abnormal lumbopelvic posture, and compensatory motion in the lumbar spine and hips during walking and other daily activities long-term.
Bone remodeling occurred in some extent in all ten ROIs. More specifically, statistically significant increases took place in ROI 1, representing the superolateral zone and in ROIs 5 and 6, representing the inferior-medial zone of the femoral neck. Lian et al. (2008) reported that valgus positioning of the neck is recommended to increase the compressive stress in the femoral head and neck . In our study, there was correlation between stem-neck angle and change in BMD in two superolateral (ROIs 2 and 3) and two larger zones (ROIs 7 and 9), which indicate that the change of stem-angle towards valgus may have some effect on loading of the femoral neck and, thus, on bone remodeling. The stem is important for proper alignment of the femoral component during the operation and the initial stability. However, as the short stem in the femoral neck is situated in cancellous bone, its importance on weight bearing and BMD needs to be researched further.
A limitation of the study is a rather short follow-up time. In other studies published the follow-up times have varied between 6-24 months [6–8]; thus, longer follow-ups with larger patient groups are needed. The strength of the study is that we have repeated the DEXA measurements four times, enabling changes of bone remodeling to be shown within the time frame. We also determined some clinical and functional outcomes of the participants when trying to identify patient-related variables to explain changes in BMD. However, the bone remodeling after HRA may be a result of many simultaneously influencing factors; thus, more research is needed to show these associations.