Morphological map of the proximal ulna bare area: a computer-assisted anatomical study in relation to olecranon...

The bare area is defined as a transverse region within the trochlear notch, serving as an optimal entry point for olecranon osteotomy due to the absence of articular cartilage coverage. However, there is limited research on the morphology and location of the bare area, and there is a lack of intuitive visual description. Thus, the purpose of this study is to delineate anatomical features of the bare area and visualize its morphology and refine the olecranon osteotomy approach.

Thirty-six cadaveric elbow joints (comprising 18 pairs) were meticulously dissected. Measurements encompassed the lateral (radial side) and medial (ulnar side) widths, proximal and distal lengths, and the distance from the corresponding dorsal cortical point of the bare area to the triceps insertion. Post-dissection, the humeral ulnar joint was realigned, followed by randomized transverse or chevron osteotomy. Subsequent computed tomography scans were conducted pre-osteotomy and post-osteotomy to delineate the shape of the bare area and osteotomy fracture line, facilitating the generation of superimposed and heat maps for visualization.

The bare area was present in all specimens, exhibiting a lateral (radial) width of 7.09 ± 4.86 mm, a medial (ulnar) width of 12.08 ± 3.66 mm, a proximal length of 15.70 ± 8.06 mm, and a distal length of 16.49 ± 7.06 mm. The distance from the triceps insertion to the corresponding dorsal cortical point of the bare area averaged 18.12 ± 3.21 mm. Notably, considerable variability was observed in both the position and shape of the bare area. Visualization through superimposed and heat maps revealed a bow-tie configuration, with the medial side wider than the lateral side, situated at the narrowest segment of the proximal ulna in the coronal plane, analogous to its waist. The superimposed map of fracture lines reveals that the fracture lines from transverse osteotomies are more concentrated than those from chevron osteotomies.

The position and shape of the bare area demonstrates notable diversity, manifesting not as a strictly transverse shape nor a consistently contiguous region. Rather, the bare area generally assumes a bow-tie configuration, rendering the conventional definition of its width along the sagittal plane inadequate and potentially misleading. Based on the typical position of the bare area, we can propose that when the precise morphology and position of a patient's bare area are unknown, targeting this region via an osteotomy from the proximal ulna's narrowest segment provides an effective approach.