Reattachment of the flexor and extensor tendons at the epicondyle in elbow instability: a biomechanical comparison of techniques

The elbow is the most commonly dislocated joint in children and the second most dislocated joint in adults with an estimated incidence of elbow dislocations in the general United States population of about 5.21 per 100,000 person-years [1, 2]. The stabilizing structures of the elbow joint are typically classified as primary, secondary, and dynamic stabilizers [3, 4]. More precisely, primary stabilizers include the bony ulno-humeral articulation, the lateral collateral ligament (LCL) complex as well as the medial collateral ligament (MCL) complex. Secondary stabilizers include the radial head, the anterior and posterior joint capsule, and the common flexor and extensor muscle origins. Finally, the biceps muscle, the brachialis muscle, the anconeus muscle, and the triceps muscle are classified as dynamic stabilizers [3, 4].

While simple elbow dislocations – defined as acute dislocations without concomitant significant fractures - may be accessible through non-operative treatment, complex dislocations involving fractures of the radial head or neck, olecranon, coronoid, humeral condyles or epicondyles typically require surgical intervention [5, 6]. Even though the majority of dislocations can be considered “simple”, “complex” cases still occur in up to 20% of patients suffering from a traumatic elbow dislocation [7]. As the postoperative results have been historically hampered by frequent stiffness with or without recurrent instability, discussions have been raised, whether to augment to the reduction with some type of external fixation or whether the length of postoperative immobilization should be prolonged [8, 9]. However, there is actual consensus in the literature that early rehabilitation following simple elbow dislocation is the best way to prevent range of motion deficits [10]. Whether an early postoperative rehabilitation is safe for the reconstructed certainly depends on the stability of the refixated structures.

Therefore, the objective of this study was to assess the biomechanical properties of a single and a double row technique for re-fixation of the common extensor muscles and common flexor muscles origin. We hypothesized that the double row technique would provide greater stability in terms of pullout strengths than the single row technique.

The most important findings of the study were that stiffness was not significantly different between the two tested techniques and that the double row technique was significantly superior to the single row technique concerning maximum load to failure. These findings confirm our hypothesis and support the use of a double row technique for re-fixation of the common flexor or extensor origin to the medial or lateral epicondyle following acute elbow dislocation.

The use of suture anchors for re-fixation of primary or secondary stabilizers of the elbow is a common and proven technique in daily clinical practice [14, 15]. Although there exists a paucity of literature concerning controlled laboratory studies evaluating different types of fixation techniques for tendon-to-bone repairs of the elbow, multiple studies already assessed the biomechanical strengths of rotator cuff repairs. Assuming the results of the shoulder to be transferable to the elbow, the ideal repair construct has to provide sufficient contact pressures at the bone-tendon-interface over the greatest possible contact area [16]. As a result, the double row repair technique has been recently evolved [16, 17]. Moreover, to avoid a functional tenodesis of the repaired tendon and thus a compromised blood supply hampering the healing, knotless and self-reinforcing repair techniques have been developed [18, 19].

The results of our controlled laboratory study support the assumption that the aforementioned findings from rotator cuff studies are valid for the elbow, too. We found the double row repair technique to be significantly stronger than the single row technique for both the common extensor and flexor origin repair. Even though reasons for this finding have not been assessed, several arguments can explain these findings. First of all, the use of an additional suture anchor has probably added further stability to the repair. Furthermore, the resultant double row construct allowed for a better distribution of the loading forces and thus were probably able to withstand significantly higher loads compared to the single row repair technique before failing [17]. Finally, a further known advantage of knotless fixation and thus a potential contributing factor to increased fixation strengths is the consistency in the fixation strengths, as previous studies have demonstrated that hand-tied knots have a high variability of strength [20, 21]. Of note, the vast majority of our constructs failed at the suture-tendon-interface with the sutures cutting out of the tendon. This mode of failure is typical for tendon-to-bone repairs and generally considered to be the weak spot of the repair [22].

Taking all of our findings into account, we believe that the double row construct is not only biomechanically but might also potentially be clinically superior to the single row construct as it allows for a reliable and early rehabilitation postoperatively. However, future comparative clinical studies have to confirm our assumption. Moreover, there exist some disadvantages of the double row repair technique in daily practice, which need to be mentioned as well. First of all, the extent of injury in case of a complex and acute elbow dislocation is most likely not limited to the flexor muscles and/or extensor muscles but also includes the primary stabilizers such as the collateral ligaments. As those might need to be re-fixated with suture anchors, too, the total amount of suture anchors used for the repair should be limited to avoid iatrogenic deterioration of the bone. Apart from that, the correct intraoperative positioning of the suture anchors gets more challenging with an increasing number of anchors used. Finally, the additional suture anchor adds surgery time and costs to the repair.

Overall, this biomechanical study provides utility by removing many external variables that may impact results, making a direct comparison of the two techniques more accurate. However, there are also inherent limitations to a cadaveric biomechanical study that cannot be controlled. The uniaxial forces applied to the common flexor or extensor tendon vertically from the humerus may not accurately reflect the dynamic loads experienced throughout a full range of motion of the elbow. More precisely, our setup did not take varus and valgus movements into account which play a substantial role in elbow dislocations. Moreover, without the contribution of healing, scarring, or muscle contractions, the measured fixation strength only simulates reconstruction immediately after surgery. Nonetheless, simulating the threshold of fixation strength immediately post-operatively may be useful information for developing appropriate rehabilitation protocols and may be useful for subsequent clinical studies.

The double row technique provides significant greater stability to the re-fixated common flexor or extensor origin to the medial or lateral epicondyle. Thus, it should be considered in the development of improved repair techniques for stabilizers of the elbow.