SLR - November 2015 - H. Litchfield
Reference: Steer RA, Smith SD, Lang A, Hohmann E, Tetsworth KD. Does Joint Architecture Influence the Nature of Intra-articular Fractures? Injury. 2015 Jul;46(7): 1299-1303.Scientific Literature Review
Reviewed By: Andrew H. Litchfield, DPM
Residency Program: Hoboken University Medical Center
Podiatric Relevance: Intra-articular fractures follow recognizable patterns throughout the body, and the foot and ankle are no exceptions. These patterns are quickly recognized on radiographs, but the true mechanics of why intra-articular fractures occur in the manner they do is often an afterthought. The authors of this paper noticed that the majority of intra-articular fractures occur in the concave side of the joint, regardless of anatomic location. In this study, the authors seek to explain the reason behind this pattern. They hypothesize that the three dimensional structure of the opposing joint surfaces is the biggest factor in determining the fracture pattern, and that subluxation at a joint may alter this pattern.
Methods: The authors performed a retrospective analysis of radiographic and CT images containing intra-articular fractures. Joints included in their analysis were the hip, knee, ankle, shoulder, wrist (radio-scapho-lunate articulation), elbow (radio-capitellar articulation) as well as the metacarpal-phalangeal, metatarsal-phalangeal and interphalangeal joints of the fingers and toes. Additional inclusion criteria consisted of patients over 18 years of age with a mechanism of injury most consistent with axial loading. Cases were excluded if the fracture involved joints other than those listed and if the mechanism of injury was unlikely to be a result of an axial load. A total of 1003 cases were reviewed to compare the proportion of concave and convex surface fractures. Also, the percentage of dislocations associated with fractures on the concave and convex surfaces were analyzed.
Results: Out of the 1003 cases reviewed by the authors, there were 102 in the ankle and 80 in the foot. Overall, 956 or 95.3 percent of fractures occurred in the concave surface of the joint. In the foot and ankle, the concave surface of the joint was fractured 91.3 percent and 97.1 percent of the time respectively. These results were statistically significant, with the risk of sustaining a fracture in the concave surface of a joint 20.8 times higher than in the convex surface. When a fracture of the convex surface did occur, it was associated with dislocation of the joint 55.3 percent of the time. The risk of sustaining a fracture in the convex surface of a joint was 18.6 times higher when joint dislocation occurred.
Conclusions: Most joints in the body, including those in the foot and ankle, can roughly be likened to having two opposing surfaces, one concave and one convex. The convex surface of the joint acts as the outside of an eggshell, able to disperse axial loads into compressive forces. The concave surface, however, is like the inside of an eggshell or a dome, and axial forces are converted to tension which is much more difficult for bone to handle. Since the same amount of force is transmitted to both sides of a joint, the concave surface will therefore fail before the convex surface with purely axial loads. This fundamental idea was confirmed by the study, with the authors stating the three dimensional architecture of the joint surfaces to be the most critical reason the majority of intra-articular fractures occurred in the concave surface. Joint dislocation, however, creates a situation in which the convex surface of a joint becomes more susceptible to fractures. The authors propose two main reasons for this: the introduction of shear forces and point loading. The authors conclude that their findings should be considered a fundamental property of intra-articular fractures. One of the admitted limitations to this study was that the mechanism of injury was determined solely from the patient’s history. Also, the density of the bone was not assessed, and could be a contributing factor to the overall strength of a joint surface.