Dual-energy X-ray absorptiometry as a measure of healing in fractures treated by intramedullary nailing

Imaging techniques for the assessment of fracture repair

Imaging of a healing fracture provides a non-invasive and often instructive reproduction of the fracture repair progress and the healing status of bone. However, the interpretation of this reproduction is often qualitative and provides only an indirect and surrogate measure of the mechanical stability of the healing fracture. Refinements of the available imaging techniques have been suggested to more accurately determine the healing status of bone. Plain radiographs provide the ability to determine the degree of bridging of the fracture gap and to quantify the amount of periosteal callus formation. Absorptiometric measures including dual X-ray absorptiometry and computed tomography provide quantitative information on the amount and the density of newly formed bone around the site of the fracture. To include the effect of spatial distribution of newly formed bone, finite element models of healing fracture can be employed to estimate its load bearing capacity. Ultrasound technology not only avoids radiation doses to the patients but also provides the ability to additionally measure vascularity in the surrounding soft tissue of the fracture and in the fracture itself.

Comparison of bone mineral parameter measurements by dual-energy x-ray absorptiometry with bone stiffness measurements as indicators of the load-bearing capacity of regenerating bone

OBJECTIVES

The objective of this study was to compare bone mineral density and content measurements by dual-energy x-ray absorptiometry with bone stiffness measurements as indicators of the load-bearing capacity of regenerating bone during distraction osteogenesis.

METHODS

The study analyzed ex vivo a set of 26 sheep bone specimens after distraction osteogenesis. Bone mineral density and bone mineral content were measured by dual-energy x-ray absorptiometry. Biomechanical parameters such as torsional, compressive, and bending stiffness were determined using a material testing system. The maximum torsional moment applied at fracture is used to determine the load-bearing capacity of the bone.

RESULTS

The correlation between stiffness measurements and the load-bearing capacity of a bone is significantly greater than the correlation between load-bearing capacity and bone mineral density and content measurements by dual-energy x-ray absorptiometry.

CONCLUSION

The results of this ex vivo study indicate that stiffness measurements are a better quantitative indicator of the load-bearing capacity of a bone and thus of bone healing than bone mineral density and content measurements by dual-energy x-ray absorptiometry. The results of the current study do not confirm previous work that showed that there is a significant correlation between bone material quality obtained by dual-energy x-ray absorptiometry and bone strength.

A finite element inverse analysis to assess functional improvement during the fracture healing process

Assessment of the restoration of load-bearing function is the central goal in the study of fracture healing process. During the fracture healing, two critical aspects affect its analysis: (1) material properties of the callus components, and (2) the spatio-temporal architecture of the callus with respect to cartilage and new bone formation. In this study, an inverse problem methodology is used which takes into account both features and yields material property estimates that can analyze the healing changes. Six stabilized fractured mouse tibias are obtained at two time points during the most active phase of the healing process, respectively 10 days (n=3), and 14 days (n=3) after fracture. Under the same displacement conditions, the inverse procedure estimations of the callus material properties are generated and compared to other fracture healing metrics. The FEA estimated property is the only metric shown to be statistically significant (p=0.0194) in detecting the changes in the stiffness that occur during the healing time points. In addition, simulation studies regarding sensitivity to initial guess and noise are presented; as well as the influence of callus architecture on the FEA estimated material property metric. The finite element model inverse analysis developed can be used to determine the effects of genetics or therapeutic manipulations on fracture healing in rodents.

Weight bearing after tibial fracture as a guide to healing

BACKGROUND

Judging when it is safe to remove an external fixator or plaster cast requires clinical and radiological assessment, both of which are subjective. Weight bearing has been shown to increase with time post-fracture and we hypothesised that it could be used as an objective measure of fracture healing.

METHODS

Ground reaction force (and hence weight bearing) and fracture stiffness were measured serially in a group of 12 patients with tibial fractures treated by external fixation. Ground reaction force was measured for both fractured and non-fractured limbs using a force plate and the fracture stiffness was measured using the Orthometer, a commercially produced device for measuring the stiffness of fractures treated by external fixation.

FINDINGS

In 10 patients who made good recoveries, prior to fixator removal, weight bearing though the injured leg was seen to approach 90% of that through the uninjured leg and the fracture stiffness exceeded 15 Nm/deg. Two patients with delayed union achieved weight bearing of 40% of normal and a fracture stiffness of less than 5 Nm/deg at 20 weeks.

INTERPRETATION

Weight bearing correlates reasonably well with fracture stiffness. It is quicker and easier to measure than fracture stiffness and potentially has relevance to other fracture fixation methods.

Outcome assessment in clinical trials of fracture-healing

Although there are numerous methods for defining fracture-healing in clinical studies, no consensus exists regarding the most valid and reliable manner for assessing union or for determining which outcomes are most important. This article summarizes and describes methods for the clinical assessment of fracture-healing and reports results from a systematic review of prevalent definitions currently used in published clinical studies. Conventional radiography and ad hoc clinical definitions continue to be the most commonly used means of assessing fracture-healing in clinical studies. Investigators must improve upon and apply more rigorous outcome assessment in clinical trials, emphasize patient-important outcomes, and report factors that may bias estimated effects.

Comparison of various types of stiffness as predictors of the load-bearing capacity of callus tissue

The ability to predict load-bearing capacity during the consolidation phase in distraction osteogenesis by non-invasive means would represent a significant advance in the management of patients undergoing such treatment. Measurements of stiffness have been suggested as a promising tool for this purpose. Although the multidimensional characteristics of bone loading in compression, bending and torsion are apparent, most previous experiments have analysed only the relationship between maximum load-bearing capacity and a single type of stiffness. We have studied how compressive, bending and torsional stiffness are related to the torsional load-bearing capacity of healing callus using a common set of samples of bone regenerate from 26 sheep treated by tibial distraction osteogenesis. Our findings showed that measurements of torsional, bending and compressive stiffness were all suitable as predictors of the load-bearing capacity of healing callus. Measurements of torsional stiffness performed slightly better than those of compressive and bending stiffness.

Quantitative and qualitative assessment of closed fracture healing using computed tomography and conventional radiography

RATIONALE AND OBJECTIVES

Development of new agents to induce fracture healing requires more sensitive methods to detect early changes in fracture repair. The aims of this study were to determine quantitative and qualitative features of fracture healing using volumetric computed tomography (CT) and to compare them with conventional radiography during the weeks following uncomplicated fractures of the appendicular skeleton.

MATERIALS AND METHODS

39 otherwise healthy men and women with acute, closed fractures of the distal radius, tibial and/or fibular malleoli, or tibial shaft, were enrolled and underwent CT and X-ray imaging at 1, 2, 4, 8, 12, and 16 (tibial shaft only) weeks post fracture. Qualitative assessment included fracture line/margins, fracture gap, external callus appearance, callus-to-cortex ratio, bridging, and radiologic union. Quantitative assessment of CT density changes (Hounsfield units [HU]) in the fracture gap was performed in a subset of 8 fracture patients using MEDx multimodality image analysis software (Sterling,VA). The analysis was performed by drawing free form regions of interest (ROI) covering the fracture gap on baseline (week 1) images and by automated registration of the follow-up images to the baseline co-ordinate system.

RESULTS

The mean time to achieve radiologic union on CT was slightly shorter than on X-rays for radial and tibial shaft fractures (7.3 vs. 8.0 weeks, P = .1). Blurring of the fracture margins and reactive sclerosis were the earliest signs of healing in both modalities. External callus formation was evident in 11 cases and was detected earlier with CT technique. Overall, CT images allowed for more complete and detailed visualization of healing compared with conventional X-rays, which were limited by cast and fixation hardware superimposition, especially in subjects with malleolar and distal radial fractures. Quantitative evaluation showed good intraobserver and interobserver reproducibility and a statistically significant correlation to qualitative changes.

CONCLUSION

Our methods of fracture healing assessment are reliable tools that are able to detect early changes in normal bone healing and may serve as useful additions to subjective image analysis in monitoring fracture healing in clinical trials. CT shows some advantages over conventional X-rays in evaluation of early fracture healing.

Complex tibial fracture outcomes following treatment with low-intensity pulsed ultrasound

A clinical study was conducted to investigate the effect of low-intensity pulsed ultrasound (US) stimulation (LIPUS) on the healing of complex tibial fractures. Thirty complex tibial fractures were randomly assigned to the treatment with LIPUS (n = 16) or by a dummy machine (sham-exposed: n = 14). The fractures were immobilized by either internal or external fixations according to the clinical indications. LIPUS was given 20 min/day for 90 days. Fracture healing was monitored by clinical, radiological, densitometric and biochemical assessments. The LIPUS-treated group showed statistically significantly better healing, as demonstrated by all assessments. Complications were minimal in the LIPUS group. There were two cases of delayed union, with one in each group. There were two cases of infection in the control group. The delayed-union cases were subsequently treated by LIPUS and the infection cases were treated with standard protocol. Fracture healing in these patients was again treated by LIPUS.

Biomechanical monitoring of healing bone based on acoustic emission technology

Acoustic emission testing is a well-established method for assessment of the mechanical integrity of general construction projects. The purpose of the current study was to investigate the usefulness of acoustic emission technology in monitoring the yield strength of healing callus during external fixation. Thirty-five patients with 39 long bones treated with external fixation were evaluated for fracture healing by monitoring load for the initiation of acoustic emission signal (yield strength) under axial loading. The major criteria for functional bone union based on acoustic emission testing were (1) no acoustic emission signal on full weightbearing, and (2) a higher estimated strength than body weight. The yield strength monitored by acoustic emission testing increased with the time of healing. The external fixator could be removed safely and successfully in 97% of the patients. Thus, the acoustic emission method has good potential as a reliable method for monitoring the mechanical status of healing bone.

Prediction of mechanical properties of healing fractures using acoustic emission

The objective of this study was to develop a non-destructive method for monitoring fracture healing with acoustic emission (AE). Experimentally produced fractures of the rat femur were tested in tension and in torsion at 4, 6, 8 and 12 weeks after fracture. AE signals were monitored during these mechanical tests. The values for load and torque at the initiation of the AE signal were defined as new mechanical parameters. The apparent density and ash density of the fracture site were also measured at each time period. Tensile strength, tensile stiffness, maximum torque and torsional stiffness of the fracture site increased with time. The AE signal was detected before complete specimen failure. Load and torque for initiation of AE increased proportionally with increasing mechanical properties. The mineral density, however, reached a plateau at 8 weeks, when callus mechanical strength was approximately 50% of control. Load for initiation of AE was strongly correlated with the strength (r = 0.98), stiffness (r = 0.88), and failure strain (r = -0.63) of the callus. Torque for initiation of AE was highly correlated with the maximum torque (r = 0.95) and torsional stiffness (r= 0.93) of the callus. The findings of the present study indicated that some mechanical properties of healing fractures could be estimated by monitoring AE signals.