With increasing rates of operative fixation of diaphyseal humeral fractures 2, 3, it is important to understand the ideal techniques to avoid failure and complications in patients undergoing operative fixation. We sought to develop a greater understanding of how different fracture characteristics may demand a deviation from historic guidelines, and how to technically optimize fixation. We observed an overall revision rate for mechanical failure of 13%. Our findings are consistent with previous reports which have observed revision rates ranging from 2–30% 6, 7.
The current study demonstrated a significant rate of aseptic mechanical failure in constructs with less than 8 cortices of proximal or distal fixation (P < 0.05). Previous reports have suggested a minimum of 6–8 cortices of fixation on either side of a humeral shaft fracture undergoing plate fixation 1, 8, 13. Our results suggest that fixation constructs with 4 bicortical screws or more of fixation on either side of the fracture had lower failure rates than those fixed with 3 screws a side. Although we cannot comment on any potential advantage of having more than 4 screws a side, surgeons are routinely using more fixation, tailored to the specific fracture type, which can often include longer constructs or multiple plates. To our knowledge, there are no studies that directly compare 6 to 8 cortices of fixation in diaphyseal humerus fracture.
When comparing proximal and distal fixation, fixation of the proximal segment may be more critical in minimizing mechanical failure. We found inadequate proximal fixation, in the form of less than 8 cortices of fixation, to be a significant predictor of failure (P = 0.011), with an eight-fold increase in the rate of mechanical failure compared to constructs with 8 or more cortices of proximal fixation (OR 7.96). Secondary analysis of failure by fracture location (proximal, middle and distal third), was also conducted. Significantly higher rates of failure were seen in patients with middle (P = 0.45) and distal third fractures (P = 0.31) with less than 8 cortices of proximal fixation. Additionally, we observed that among those with middle third shaft fractures, failure occurred frequently in the form of proximal screw pull out (Fig. 1D & E). We hypothesize these findings are due to features unique to the proximal humerus. Firstly, the proximal humeral diaphysis is directly enacted on by multiple deforming forces including coronal and sagittal plane forces. In addition, it is uniquely subjected to rotational torque forces applied by the teres major and minor, infraspinatus, subscapularis, pectoralis major and latissimus dorsi 14. Adequate fixation must overcome and withstand these multiplanar forces on the proximal segment. Secondly, it has been well documented that the proximal humeral cortical diaphysis thickness is variable and has decreased cortical thickness in osteoporotic bone 15. As a result, special attention must be paid to the type and amount of fixation in that segment to avoid proximal screw pull out.
Regarding proximal third diaphyseal fractures, we failed to see a significant difference between those with 8 or more cortices of proximal fixation vs those with less than 8 cortices (P = 1.0). We attribute these equivocal findings to the preference of proximal humerus locking construct for fixation of these fractures, in which unicortical locking screws provide a mechanical advantage in a shorter proximal segment and allow for higher screw density. These constructs cannot be directly compared to non-locking plates used in more distal fractures. A higher rate of mechanical failure was also seen in distal third diaphyseal fractures with less than 8 cortices of distal fixation (P = 0.046). As distal fixation was not found to be a significant predictor of failure after regression analysis, the significance of this finding is indeterminant and we cannot draw conclusions regarding the number of cortices of distal fixation. Thus, 8 cortices of proximal fixation may be recommended, especially in middle and distal third diaphyseal fractures fixed with straight plates without proximal locking extension. It is important to ensure screws are truly bicortical if intended, as demonstrated by the patient in Fig. 1C, who went on to aseptic mechanical failure, with post-operative radiographs demonstrating that the screws were not fully engaged in the distal cortex.
There are several limitations to our study. The limited sample size may have resulted in a type II error. Although a large volume of patients underwent operative management of a diaphyseal humerus fracture during the study period, a significant proportion did not meet inclusion criteria. The principal reason for exclusion in the majority of cases was lack of follow-up to radiographic and clinical union. As a referral center for high volume trauma, many patients were either lost to follow-up, or transferred back to the referring centers for long-term follow-up after acute management of their injuries. As a result, we were also unable to account for patients who may have experienced fixation failure and presented to another institution for care. The remainder of patients who were eligible were followed until radiographic union and discharged from follow-up. We felt that this was adequate and did not pursue follow-up to one year, as our endpoint was defined as mechanical failure or discharge. The retrospective nature of the current study is also a limitation as it is subject to the inherent biases associated with this study design. Finally, we were unable to locate a comprehensive list of patient comorbidities as often these were not documented. Despite this, we did not find any significant demographic differences between the two groups, thereby improving the generalizability of our results.