Elsevier

Clinical Biomechanics

Volume 20, Issue 3, March 2005, Pages 277-282
Clinical Biomechanics

A quantitative and qualitative analysis of bone remodelling around custom uncemented femoral stems: a five-year DEXA follow-up

https://doi.org/10.1016/j.clinbiomech.2004.10.007Get rights and content

Abstract

Background. After total hip replacement bone remodelling is determinant for the implant survival. This study asked whether the immediate postoperative amount and distribution of bone can predict the changes in bone amount and distribution after five years in the case of younger patients with custom uncemented implant.

Methods. We investigated 17 hips with a mean follow-up of 5.1 years. The average patient age at follow-up was 48.5 years. We used immediate postoperative and five-year dual energy X-ray absorptiometry measurements of bone mineral content, density, and projected bone area in seven local zones forming a partition of the upper femur. A correlation was sought between the preoperative variables and the five-year variation. Qualitative bone remodelling was analysed considering each local zone in a density-area plane. Based on geometrical considerations, we introduced an index of structural remodelling, which expresses the preponderance of internal remodelling against external modelling.

Findings. The bone mineral content at operation correlated significantly with its relative decrease locally laterally mid-proximally and medially (P < 0.01), distally, and globally for the femur (P < 0.05). For the bone mineral density, the correlation was significant distally, medially and globally (P < 0.05). The bone projected area shows only significant correlation laterally mid-proximally (P < 0.01). The index of structural remodelling was significantly positive (P < 0.01) in all local zones and was independent of the initial bone amount and structure (P > 0.1).

Interpretation. High bone mineral content at operation correlates significantly with periprosthetic bone loss after five years in younger patients with cementless custom femoral implant. Independently of the net bone mineral content balance, external modelling is stronger than internal remodelling in all local zones.

Introduction

The qualitative aspect of periprosthetic bone remodelling has been extensively reported in the literature. Follow-up studies for most implant types describe qualitatively the frequency and distribution in gain or loss of bone mass, and the presence of radiolucencies, or osteolytic lesions (Clohisy and Harris, 1999, Eingartner et al., 2000, Keisu et al., 2001).

A number of studies use dual energy x-ray absorptiometry (DEXA) to monitor changes in bone mineral content (BMC), bone mineral density (BMD), and projected bone area after insertion of different implants (Engh et al., 1992, Kilgus et al., 1993). Cohen and Rushton (1995) showed, using two different implant types, that the in vivo mean coefficient of variation of the BMD varied between 2.7% and 3.4% for repeated measurements in the same conditions. The reliability of DEXA for the measurement of changes in bone mass around prostheses has also been documented by others (Kröger et al., 1996, Larnach et al., 1992). Primarily, the measurement error follows from the variations in the position of the patient during scanning. Mortimer et al. (1996) showed that the BMD and the BMC varied within 5% between 15° internal and 15° external rotation.

Sychterz and Engh (1996) reported a positive correlation between initially low BMC and high postoperative bone loss. However, these results are based on DEXA measurements of BMC in bones retrieved from elderly patients post mortem previously equipped with a straight extensively coated endoprosthesis. A recent in vivo study (Rahmy et al., 2004) reported the same relation based on measurements of BMD.

Young patients show a different remodelling activity both at the cellular (Groessner-Schreiber et al., 1992) and at the macroscopic level (Brockstedt et al., 1991). Their more active lifestyle, often mentioned as a cause of implant loosening (Dorr et al., 1997), also represents increased loads on the femur and thus a stimulus for gain in bone mass. Together these findings suggest that younger patients might have a different remodelling pattern after the insertion of orthopaedic implants.

Follow-up studies of bone remodelling only report the evolution of BMC or BMD as a function of time (Georges et al., 2002, Massari et al., 1995, Kröger et al., 1996). These data reflect only either the total remodelling or the internal remodelling and these concepts are often confused. In this paper, we call internal remodelling, the reorganization of cancellous bone resulting in changes of bone density, while we call external modelling processes occurring at the external surface of bone resulting in geometrical changes. Total remodelling sometimes simply called remodelling denotes then both processes. In addition to the redistribution of bone mineral content (total remodelling) also the external modelling or the change in femoral shape (surface modelling) is of great interest in order to understand the adjustment of bone to a new mechanical environment.

Therefore, the objective of the present study was twofold. First, we examined whether a predictive relationship could be established between pre-operative bone quality and bone remodelling in order to help identifying patients at risk for increased bone loss and, secondly, we mapped changes in bone structure by comparing external modelling with internal remodelling. For this purpose we hypothesized that BMC, projected bone area, and BMD could be quantitative predictors of bone remodelling in the proximal femur in younger patients after insertion of a custom implant, then we tried to describe changes in the bone structure based on DEXA output.

Section snippets

Patients

Nine men and seven women (17 hips) were operated between June 1997 and June 1998 with a custom uncemented femoral implant (Unique, Scandinavian Customized Prostheses, Trondheim, Norway). The mean patient age at operation was 43.4 years (ranging from 20 to 59 years) and the mean follow-up time was 5.1 years (ranging from 4.6 to 5.6 years). Two experienced surgeons performed all hip replacements. All measurements were performed with one DEXA machine (Hologic QDR4500, Bedford, USA) and by one

Quantitative analysis

All correlation coefficients were negative indicating that the relative bone loss after five years is more important for patients with higher immediate postoperative bone mass. High BMC postoperatively was significantly correlated to high relative BMC loss after five years (P < 0.05) in the Gruen zone R2, R4–R7 and globally. The slopes of the linear regressions are reported in Table 1. The projected bone area postoperatively showed a poor correlation with its relative variation, which was

Discussion

This study shows that bone mineral content can be a quantitative predictor of bone remodelling for younger patient with custom, uncemented femoral implants. It shows furthermore that the higher the BMC at operation, the higher the relative bone loss. Femurs 20% below the average BMC will experience an 8% bone gain while femurs 20% above the average BMC will experience a 10% bone loss. The lack of correlation for the projected bone area and the weaker correlation obtained with the BMD may be

Acknowledgment

Professor Gunnar Leivseth MD, PhD, Department of Neuroscience, Faculty of Medicine, Norwegian University of Science and Technology, gave valuable comments on this article.

References (18)

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