Using a laser scanner, we created cartilage models from cadaveric elbows and quantified the distribution of cartilage thickness throughout the elbow. We found that thicker cartilage areas in individual elbow bones gathered at the joint center, facing each other. The combined findings revealed that the thicker cartilage regions were associated with articulation of the 3 joints of the elbow (Fig. 10).
First, regarding the radiocapitellar joint, our findings showed that the cartilage thickness on the rim circumference within the dish of the radial head was significantly greater than that on all of the inner circumferences, similar to the results reported by Yeung et al.35. The articular zone cartilage was thicker than the safe zone cartilage in the outer circumferences and, facing this area, the cartilage in the intermediate region of the distal part of the humerus was thicker as well. These findings indicated that thicker cartilage along the rim may help to increase radiocapitellar stability secondary to concavity compression onto the concave articular dish1,35,39-41. Second, thicker cartilage in the ulnotrochlear joint was distributed from the anterolateral edge of the coronoid to the anteroproximal edge of the proximal sigmoid notch and faced the intermediate region of the distal part of the humerus. This asymmetrically distributed pattern was in line with the knowledge that the ulnotrochlear joint is an asymmetrical trochoid joint having an obliquity to the axis of rotation42. Third, in the proximal radioulnar joint, there was a thicker cartilage area in the anteroproximal edge of the proximal sigmoid notch, which was faced by the articular zone cartilage on the proximal circumferences around the side.
Taken together, our findings indicated that the thicker cartilage regions in individual bones gathered at the joint center. We believe that this spatial “cartilage gathering” is a novel anatomical finding, which led us to speculate that the cartilage might thicken in the area where individual bones gather, as if to fill the joint gap to fit the complex structures and stabilize the joint43,44. In terms of flexion-extension motion, articulation reportedly contributes more to elbow stability in the flexed position42. Our results showed that cartilage thickens in the anterior zone of the distal part of the humerus and in the coronoid of the trochlear notch, where the closest relationship of the cartilage surfaces during elbow flexion provides a certain degree of joint stability45-48. These trends should be constant in this age range as long as normal cartilaginous conditions exist.
Our findings are clinically relevant to anatomical implant and prosthesis designs. Considering the spatial variability of cartilage in implant design can replicate normal human articular surface anatomy, thus restoring joint kinematics, maximizing contact area with the native articular surfaces, and potentially leading to improved postoperative outcomes35,49. Several studies have experimentally revealed that subtle change in prosthetic configuration affects joint biomechanics1,39-41; thus, small differences in cartilage thickness would be clinically meaningful. This concept is applicable to radial head replacement, distal humeral hemiarthroplasty, and capitellar arthroplasty in several clinical scenarios, including severe fractures, isolated degeneration at the radiocapitellar joint, and osteonecrosis5. Next, anatomical knowledge of cartilage thickness is useful for treating fractures. Regarding the development of arthritis, joints are believed not to tolerate step-offs of more than the cartilage thickness50-53; therefore, fractures occurring in thinner cartilage regions may be more susceptible to posttraumatic arthritis, potentially lowering the threshold for surgical treatment compared to fractures in regions with thicker cartilage.
Furthermore, an understanding of the spatial cartilage anatomy may help to elucidate elbow osteoarthritis. Unpredictable incongruity resulting from articular roughness with joint instability is assumed to contribute to osteoarthritic progression54; nevertheless, there is a paucity of literature that supports these theories. Our results not only provide important information to predict the articular roughness such as posttraumatic step-offs and surface irregularities associated with degenerative conditions but also provide a possible explanation regarding joint instability42,45-48.
The present study had some limitations. First, we used formalin-embalmed cadavers. Formalin fixation can lead to dehydration of the cartilage; however, several researchers have reported that formalin fixation has no measurable effects on cartilage thickness or on geometric configuration within a joint21,23,25,31,55,56. Furthermore, our measurements corresponded with those in previous in vivo cartilage studies16-19. Second, a true difference may have been overlooked due to the relatively small sample size and the study design assuming a large effect size. Also, the subjects were sampled without considering potential differences in age and sex.
In conclusion, we identified regional variations in cartilage thickness of the distal part of the humerus, proximal part of the ulna, and radial head in the elderly. Additionally, we found that thicker cartilage of individual bones gathered at the elbow joint center (“cartilage gathering”), which we believe to be a novel anatomical finding. Our findings provide additional insight that is invaluable for elbow surgery to restore normal function.
Supporting material provided by the authors is posted with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJSOA/A109).
Note: The authors would like to thank Ryoji Nakao and Taro Adachi for their skillful technical support and excellent contributions to this study. They would also like to acknowledge Yasunobu Nakamura for his statistical support.
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