Cortical and medullary morphology of the tibia

Abstract

Bone resorption caused by stress shielding and insufficient bone‐implant contact continues to be problematic for orthopedic endoprostheses that utilize osseointegration (OI) for skeletal fixation. Morphologic analyses have helped combat this issue by defining anatomic parameters to optimize endoprosthesis loading by maximizing bone‐implant contact. These studies have not typically included diaphyseal medullary morphology, as this region is not pertinent to total joint replacement. To the contrary, percutaneous OI endoprostheses for prosthetic limb attachment are placed in the diaphysis of the long bone. This study examined the cortical and medullary morphology of 116 fresh‐frozen human cadaveric tibia using computed tomography. Anatomic landmarks were selected and custom MATLAB scripts were used to analyze the cross‐sectional cortical and medullary morphology normalized to biomechanical length (BML). BML measured the distance between the tibial plateau and the tibial plafond. Properties such as cortical thickness, medullary diameter, and circularity of the medullary canal were quantified. We tested the influence of sex and laterality on morphology, and examined variations along the length of the bone. Results showed that while both sex and laterality impacted the location of anatomic landmarks, only sex influenced cross‐sectional morphology. Overall, morphology significantly affected shape along the length of the bone for all examined properties except medullary circularity. This analysis found that distal to 35% BML, the canal is conducive to a circular implant, with medullary diameter ranging from 13 to 32 mm between 20 and 80% BML. A large size range is necessary for sufficient implant contact in order to accommodate residual limb length after amputation.

Virtual implantation technique to estimate endoprosthetic contact of percutaneous osseointegrated devices in the tibia

“…Each region presents its own challenges and may require unique approaches. Unlike the humerus where we see a consistent medullary flare as you move proximally [16], the tibia has a distal convergence, diaphyseal straight region, and proximal flaring [15]. The previous approaches that allow for a single, scaled endoprosthesis for any residual limb length are likely not the best for the changing nature of the tibia.…”

“…A paired, two-means power analysis (α = 0.05, power = 0.8, effect size ≥0.6) of variation in mean cortical thickness of the tibia was conducted to determine the sample size necessary to capture cortical thickness variation in the population as this is an important measure in endoprosthesis fit. Mean cortical thickness at adjacent residual lengths of 20, 35, 50, 65, and 80% estimated a maximum population of 25 specimens (50 to 65% was omitted because there was not a statistical difference in cortical thickness between these lengths) [15]. A total of 27 fresh-frozen, unpaired tibias were selected for validation procedures.…”

“…Axial CT scans were collected (Siemens SOMATOM Definition Flash scanner, 120kVp, 100mAs, 512 × 512 acquisition matrix, 1-mm slice thickness). CT scans were reconstructed to generate 3D models (MIMICS v21.0, Materialise, USA, Plymouth, MI) at three points during the surgical process of implanting an endoprosthesis with conical geometry: 1) isolated intact bones with soft tissue removed, 2) after amputation at the 40% residual length (Figure 1) [15], and 3) after surgical placement of the endoprosthesis but before final impaction. Scan 1 was segmented using semi-automatic thresholding to identify cortical and medullary boundaries.…”

“…Scan 1 was segmented using semi-automatic thresholding to identify cortical and medullary boundaries. This segmentation was decimated and smoothed to reduce aliasing artifact by performing three smoothing iterations using a smooth factor of 0.5 (MIMICS) [15]. Scan 2 aligned the previous segmentation to the bone in this scanner and cropped the segmentation at the distal osteotomy using thresholding around the distal end.…”

“…The tibia has a unique anatomic morphology with an hourglass shaped medullary canal that has thin cortex on the proximal and distal ends and thicker cortex in the middle [15]. Each region presents its own challenges and may require unique approaches.…”