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

As a cadaveric study, our institution does not require Institutional Review Board (IRB) approval. The study was performed using 12 fresh-frozen, human cadaveric humeri of male donors only, which were donated to our research laboratory. Radial head compression tests were performed to exclude specimens with osteoporosis. More precisely, as mechanical stability of the radial head is known to correlate with bone quality, static axial compression load was applied on the cartilage surface until breakage with a speed of 10 mm/min and subsequently a load-over-displacement analysis performed [11, 12]. To ensure equal bone quality before testing, all specimen with significant deviations in the mean load-over-displacement curve were discarded. All specimens were less than 65 years of age (mean, 55.6 years; standard deviation (SD) 12.0 years), with no history of elbow injury, surgery, or anatomic abnormality and randomized into one of the two groups. Specimens were stored at − 20 °C and thawed at room temperature for 24 h before preparation. The humerus was disarticulated from the ulna and radial bone, and all soft tissue (including the collateral ligaments) except the common flexor origin (consisting of the flexor carpi radialis muscle, the flexor carpi ulnaris muscle, the palmaris longus muscle and the flexor digitorum superficialis muscle) and common extensor origin (consisting of the extensor carpi ulnaris muscle, the extensor carpi radialis brevis muscle, the extensor digitorum muscle, and the extensor digiti minimi muscle) was removed. The humerus was then potted in plaster (Moldasynth, Heraeus Kulzer GmbH, Hanau, Germany) to preserve the position during testing. Care was taken to keep an exact distance of 5 cm from the plaster to the most distal point of the humerus for each specimen (Fig. 1).

Two orthopaedic surgeons (Philipp Proier and Andreas Lenich) performed all re-fixations of the extensors and flexors. Two different techniques were used to test fixation strength using a single row or double row technique. Prior to the placing of the anchors, the common flexors and extensors including the cortical bone of its origins were removed.

The single row construct (Fig. 2) consisted of one single- and one double-loaded 3.0 mm suture anchor (SutureTak, Arthrex, Inc., Naples, FL) with 2–0 fiber wires to secure the common tendons origin to the medial or lateral humeral bone. With the use of a suitable drill, two holes were positioned like follows: The double-loaded anchor was routinely placed 1 cm proximal to the cartilage-bone-border in the extended axis of the humeral shaft with the same distance to the anterior and posterior joint surface. The single-loaded anchor was subsequently placed 1 cm proximal to the first anchor in the same axis. The six suture limbs of both the double-loaded and single-loaded anchor were shuttled through the common tendon in a mattress technique leaving an approximately 1 cm gap from the tendons margin. Finally, each pair of suture limbs was tied down using seven alternating half hitches and the sutures were cut.

The double row construct (Fig. 1) was similar to the single row construct with the only difference that all six suture limbs of both previously positioned 3.0 mm suture anchors were loaded into the eyelet of a 3.5 mm knotless suture anchor (SwiveLock, Arthrex, Inc., Naples, FL) after tying the knots. Subsequently, a bone socket was created with a punch 1 cm posterior and 0.5 cm proximal to the most proximal anchor of the single row anchors. The eyelet of the anchor was brought to the edge of the socket and the limbs of sutures were individually tensioned. The eyelet was then advanced into the socket until the anchor body contacted the bone, effectively tensioning the suture limbs. Once the anatomy of the common extensor or flexor footprint was restored, the body of the anchor was advanced clockwise into the bone socket to secure the sutures.

Each construct was biomechanically assed using a dynamic tensile testing machine (Instron ElectroPuls E10000, Instron Systems, Norwood, MA). Before clamping the muscles in a custom fixture approximately 7 cm from the common tendons margin (Fig. 2), the clamps were treated with dry ice to prevent muscle slippage within the fixture during testing. The embedded humerus was securely fixed to the stationary base of the tensile testing machine. After preloading the muscles to 10 N (extensors) or 15 N (flexors), care was taken to ensure that the common tendon was aligned vertically (Fig. 2). The preload of the muscles was defined to be 10% of the natural load of the common extensors or flexors which has been described to be about 100 N for the extensors and 150 N for the flexors [13]. Each construct was cyclically loaded at 1 Hz in 10 steps for five minutes each. The baseline load was 15 N for the extensors and 10 N for the flexors with a stepwise 15 N (extensors) or 10 N (flexors) increase after each step. If the construct was still intact after the 10 steps of cyclic loading, it was pulled to failure at 60 mm/min. Failure was defined as suture breakage or any perceived movement of the implanted anchors. Failure mode was observed and defined in each case by two reviewers. Stiffness as well as maximum load during the pull to failure and mode of failure were recorded and calculated. Stiffness of the repair was calculated as the slope of the load-versus-displacement curve at pull-to-failure (PTF) or the final cycle of cyclic loading if PTF was not reached.

An a priori power calculation was conducted and the usage of six specimens per group was found to be sufficient to detect an effect size of d = 1.2 with 80% statistical power. All continuous variables were not observed to be skewed or over dispersed, so parametric testing methods were used. Thus, t-test models were built to compare the two groups. All statistical analyses and graphics were produced using the statistical program SigmaPlot, version 13.0 (Systat, San Jose, CA).