RESUMO
Background: Gold standard for determining bone density as a surrogate parameter of bone quality is measurement of bone mineral density (BMD) by dual energy X-ray absorptiometry (DXA), most commonly performed on the lumbar spine (L1-L4). Computed tomography (CT) data are often available for surgical planning prior to spine procedures, but currently this information is not standardized for bone quality assessment. Besides, measuring the Hounsfield-Units (HU) is also of great importance in the context of biomechanical studies. This in vitro study aims in comparing BMD from DXA and HU based on diagnostic CT scans. In addition, methods are presented to quantify local density variations within bones. Methods: One hundred and seventy-six vertebrae (L1-L4) from 44 body donors (age 84.0±8.7 years) were studied. DXA measurements were obtained on the complete vertebrae to determine BMD, as well as axial CT scans with a slice thickness of 1 mm. Using Mimics Innovation Suite image processing software (Materialise NV, Leuven, Belgium), two volumes (whole vertebra vs. spongious bone) were formed for each vertebra, which in turn were divided in their left and right sides. From these total of six volumes, the respective mean HU was determined. HU of the whole vertebra and just spongious HU were compared with the BMD of the corresponding vertebrae. Side specific differences were calculated as relative values. Results: Whole bone and spongious HU correlated significantly (P>0.001; α=0.01) with BMD. A positive linear correlation was found, which was more pronounced for whole bone HU (R=0.72) than for spongious HU (R=0.62). When comparing the left and right sides within each vertebra, the HU was found to be 10% larger on average on one side compared to the opposite side. In some cases, the difference of left and right spongious bone can be up to 170%. There is a tendency for the side comparison to be larger for the spongious HU than for the whole vertebra. Conclusions: Determination of HU from clinical CT scans is an important tool for assessing bone quality, primarily by including the cortical portion in the calculation of HU. Unlike BMD, HU can be used to distinguish precisely between individual regions. Some of the very large side-specific gradients of the HU indicate an enormous application potential for preoperative patient-specific planning.
RESUMO
BACKGROUND: Atraumatic femoral head necrosis is a rare pathological change of the femoral head. It is characterized by local necrosis of the cancellous bone as a result of reduced blood supply to the bone. Even today it remains unclear how to assess the hardness of the necrosis, whether it is soft tissue that is easily removed, or hard tissue that is difficult to resect. METHODS: Femoral heads with primary coxarthrosis were selected as a comparison group. For this purpose, 49 femoral heads obtained during total hip arthroplasty surgery with either condition (23 femoral head necrosis, 26 coxarthrosis) were transferred to the testing laboratory in fresh condition. Cylindrical specimens were obtained using a tenon cutter along the main trabecular load direction in the subchondral region of the femoral head. Additionally, thin bone slices were extracted proximal and distal to the specimens for density measurements. Brass plates were glued to the circular surfaces of the specimens. After curing of the adhesive, the specimens were mounted in the testing machine and destructive uniaxial compression tests were conducted. FINDINGS: The recorded mean compressive strengths and elastic moduli were almost identical for both groups, but the necrosis group showed significantly higher data scattering and range regarding the elastic modulus. The mean density of the coxarthrosis specimens was significantly higher than that of the necrotic specimens. INTERPRETATION: The mechanical properties of cancellous bone vary considerably in the presence of femoral head necrosis. The existence of hard necrosis implies a potential challenge regarding the clinical resection of these tissues.