Introduction

There is no published series described change in bone mineral density (BMD) after ankle replacement. We present the results of a prospective study examining the effect of total ankle replacement (TAR) upon local bone mineral density (BMD).

Introduction: There is limited literature available looking into circumstances surrounding the development of stress fracture of the medial and lateral malleoli after ankle replacement. We present the preliminary results of a prospective study examining the effect of ankle replacement upon local bone mineral density and the phenomenon of stress shielding.

Aim: To assess the effect of ankle replacement loading of the medial and lateral malleoli, by analysing the BMD of the medial and lateral malleoli before and after Mobility total ankle replacement.

Methodology: Ten consecutive patients undergoing Mobility total ankle replacement for osteoarthritis had pre-operative bone densitometry scans of the ankle, repeated at 6 and 12 months after surgery. The bone mineral density of a 2 cm square area within the medial malleolus and lateral malleolus was measured. The pre-operative and post-operative bone densitometry scans were compared. The relation between the alignment of the tibial component and the bone mineral density of the malleoli was also analysed.

Results: The mean preoperative BMD within the medial malleolus improved from 0.58g/cm2 to mean 6 months postoperative BMD of 0.59g/cm2 and 0.60g/cm2 at 12 months. The mean preoperative BMD within the lateral malleolus decreased from 0.40g/cm2 to a mean 6 months postoperative BMD of 0.34g/cm2. However the BMD over the lateral malleolus increased to 0.36g/cm2 at 12 months. The mean alignment of the tibial component was 88.5° varus (85° varus to 94° valgus). There was no correlation between the alignment of the tibial component and the bone mineral density on the medial malleolus (r = 0.09, p = 0.865).

Conclusion: The absence of stress shielding around the medial malleolus indicates that TAR implanted within the accepted limits for implant alignment, load the medial malleolus. However, there was stress shielding over the lateral malleolus resulting in decreased BMD in the lateral malleolus.

There is limited literature available looking into circumstances surrounding the development of stress fracture of the medial and lateral malleoli after ankle replacement. We present the preliminary results of a prospective study examining the effect of ankle replacement upon local bone mineral density and the phenomenon of stress shielding.

We aimed to assess the effect of ankle replacement loading of the medial and lateral malleoli, by analysing the Bone Mineral Density (BMD) of the medial and lateral malleoli before and after Mobility total ankle replacement.

Ten consecutive patients undergoing Mobility total ankle replacement for osteoarthritis had pre-operative bone densitometry scans of the ankle, repeated at 6 months after surgery. The bone mineral density of a 2 cm square area within the medial malleolus and lateral malleolus was measured. The pre-operative and postoperative bone densitometry scans were compared. The relation between the alignment of the tibial component and the bone mineral density of the malleoli was also analysed.

The mean preoperative BMD within the medial malleolus improved from 0.57g/cm2 to mean 6 months postoperative BMD of 0.62g/cm2. The mean preoperative BMD within the lateral malleolus decreased from 0.39g/cm2 to a mean 6 months postoperative of 0.33g/cm2. The mean alignment of the tibial component was 88.50 varus (range 850 varus to 940 valgus). However, there was no correlation between the alignment of the tibial component and the bone mineral density on the medial malleolus (r = 0.09, p = 0.865).

The absence of stress shielding around the medial malleolus indicates that ankle replacements implanted within the accepted limits for implant alignment, load the medial malleolus. However, there was stress shielding over the lateral malleolus resulting in decreased BMD in the lateral malleolus.

This study aimed to determine the prevalence of osteoporosis in patients awaiting hip and knee replacement for osteoarthritis and to review them two years later to determine the changes in bone density following joint replacement.

Patients aged between 65 and 80 years awaiting total hip or knee replacement were invited to participate. Lumbar spine, bilateral femoral and forearm bone mineral density (BMD) measurements were obtained using dual energy x-ray absorptiometry. BMD values were standardised using previously published T-scores and Z-scores. To assess clinical status, patients completed a questionnaire including the Western Ontario and McMaster University OA Index (WOMAC). All measurements were repeated at two-years.

Participants included 199 patients (84 hips and 115 knees) with a mean age of 72 years (SD 4.0) and were predominantly female (hips 67%, knees 50%). At baseline 46/199 (23%) patients (39 females) had evidence of osteoporosis (WHO definition) at one or more sites with the highest prevalence at the forearm (14%). At two-years 144 patients attended for review with 128 having undergone hip (56) or knee (72) replacement. At this review 39/144 (27%) patients (33 females) had evidence of osteoporosis at one or more sites with the highest prevalence at the forearm (22%). The greatest bone loss occurred at the forearm with median BMD change of minus 4% for females (25th percentile minus 7.3%, 75th percentile minus 1.9%) and minus 2.9% for males (25th percentile minus 4.6%, 75th percentile minus 1.1%). There was a significant difference in WOMAC Pain scores at follow-up between the osteo-porotic and non-osteoporotic knee patients (67 versus 81, p=0.002) indicating that osteoporotic patients had greater knee pain.

We have identified the forearm as not only the site with the highest prevalence of osteoporosis but also the greatest bone loss at follow-up. Further evaluation of forearm bone density measurements in the preopera-tive assessment and follow-up of patients awaiting joint replacement for hip and knee OA is required. Larger studies are needed to confirm our finding that the presence of osteoporosis is predictive of worse patient-reported outcomes of knee replacement.

Dual Energy X-ray absorption (DEXA) has been used to measure bone mineral density (BMD) around total hip prostheses. With the recent increase in the use of metal on metal hip resurfacing, such as the Birmingham Hip Resurfacing (BHR), there has been renewed concern over per prosthetic femoral neck fracture and implant loosening. DEXA quantitatively measures bone mineral density and therefore could predict impending loosening and fracture. To the best of our knowledge, there are no recorded studies assessing BMD around metal-on-metal hip resurfacings such as the BHR. Our intention was to produce a reliable method of measuring bone density around a metal-on-metal hip resurfacing, such as the BHR, prior to a prospective study.

We performed DEXA scans on five patients (7 BHR’s), who had undergone resurfacing with the BHR within the last two years, using the Hologic QDR 45000A scanner. Each BHR was scanned twice on the same day with complete patient repositioning between scans. We analysed the data with the Hologic prosthetic hip (v 8.26a: 3) scan analysis software (operating software 9.80D) by identifying a variable number of same-sized regions of interest (ROI) within the femoral neck. These ROI’s were derived from an inter-trochanteric line and the axis of the BHR stem in the femoral neck. Each of the 14 scans was analysed twice, by three of the authors independently; with at least one week between repeat analysis by the same observer. Statistical analysis was carried out by the local University Department of Statistics.

The variation within the same ROI in a given BHR was 0.00353, whereas the variation between all ROI’s was 1.155. The intraclass-correlation was 0.997 (i.e. the correlation between any two assessments of one ROI) with an overall coefficient of variation of 5%. The variation between the two scans for each BHR and between the three assessors was not significant (p=0.87 and p=0.42 respectively). The mean BMD of the individual ROI’s, between the two assessments of the same scans by the same assessor (i.e. intra-observer variation) was lower on the second assessment by 0.0214gcm⁻² (SD=0.0025) representing 0.5% mean density for all ROI’s. This difference was statistically significant (p< 0.001).

This method demonstrates excellent reproducibility of the method. Inter-scan and inter-observer variation was so negligible that a tiny intra-observer variation of 0.5% (of mean bone density) became statistically significant (p< 0.001), despite it making no difference to the overall intraclass-correlation. Statistical advice suggested that this very small difference in mean density (intra-observer) only reached significance due to the highly sensitive measurements and excellent reproducibility.

We have designed and demonstrated a safe, non-invasive and highly reproducible method for scanning BHR implants in vivo using DEXA. We will now use this method to prospectively study our BHR population to detect impending loosening or fracture.