Background: With the ever-increasing ageing population minimally invasive techniques such as interlocking nailing for the stabilization of fractures have become standard of care. On the other hand, joint replacement is becoming clearly the focus of interest of many surgeons dealing with the geriatric population. Both surgical techniques require detailed information about the internal structure and geometry of the medullary canal.
Purpose: Few publications provide data on the dimension and shape of the humeral medullary canal. The available data show a high variability and describe either the proximal or the distal portion of the canal. Our purpose was to provide reliable data on the form and shape of the entire medullary canal.
Materials and Methods: 22 humeral bone from elderly donors (mean age 74 years) from the anatomical institute were evaluated. Of these samples, 12 were male and 10 were female. Right and left were equally distributed. The canals of the bone were cleaned and filled with a radiopaque paste. Twelve plane x-rays were taken from each bone (axially rotating the bone in steps of 15 degrees). A virtual longitudinal middle axis was built in each projection and the boarder of the medullary cavity was measured along the medullary canal in steps of 10% of its respective length. Cross sections at each measuring point were calculated using trigonometric algorithms to reconstruct the geometry and dimensions of the canal.
Results: The average canal length was 23,1 cm (mean values, n=22) and correlated with the total length of the humerus. The evaluated section of the medullary canal in our study measured 19,6 cm (60% of the total humeral length). The frontal diameter at the proximal cross section was measured as 17,3 mm and the lateral diameter as 16,1 mm (mean values, n=22). The diameter of the approximately circular cross-sections decreases continuously from proximal to distal and measures 10,6 (frontal) and 9,6 (lateral) mm at the end of the medullary canal.
Conclusion: The canal illustrates a carrot or funnel shape configuration in all planes in accordance with previous findings. Beyond that we were able to show that the cross-section of the humeral channel is almost circular in all sections. A retroversion of the most proximal section of the canal was described by some authors. In our measurement we excluded the variable extension of the medullary canal into the humeral head and could show a straight geometry at the proximal 50-60% of the evaluated canal in relation of a central canal axis. Our data confirm an angulation or bend at the distal 40% of the canal that was also seen in previous investigations.
Clinical Relevance: The characteristic carrot or funnel shaped medullary canal of the humerus has impact on either the ante- and retrograde nailing technique for fracture stabilization or on the design of prosthesis stems in shoulder arthroplasty. The absence of a relevant bending or angulation of the canal in the proximal 50% to 60% may allow for the design of better fitting prosthesis stems in future product developments.