Biomechanical Evaluation of FDP Tendon Repair

The biomechanical testing setup and the data acquisition protocol were adopted from our previous investigation^{31 (link)} evaluating repair techniques of type III FDP tendon avulsion injuries, seeming most suitable for the investigation of biomechanical characteristics of type IV injury repairs as well. To establish a firm hold of the specimens and to enable visualisation of the dynamics of the repair throughout biomechanical testing we used a distal phalanx (DP) holding cylinder mounted on an electromechanical tensile testing machine (Zwick Z050, ZwickRoell GmbH, Ulm, Germany) equipped with a 1 kN load cell. Prior to testing the load cell was checked for its accuracy in the low load region using calibrated weights. In the range from 0.1 to 5 N the maximum deviation was 7 mN. Following repair, each specimen was placed into the DP cylinder allowing full exposure of the repair through its semicircumferential window. Using common white plaster as a fixation material, all repair components remained unaffected, thus preventing unintended rotation or tilt of the specimen within the cylinder. A ball-joint linking the tensile testing machine to the cylinder ensured an optimal alignment of the specimen-repair construct along the longitudinal axis of the distal phalanx and the reattached tendon when strain was applied, preventing bias by translational forces to the repair site. Seven centimetres proximal to the repair, the tendon was retained using a standard tensile clamp. The articular line of the distal phalanx, of the avulsion fragment, as well as the tendon at the corresponding articular line level were marked with a blue felt tip pen under a baseline preload of 2 N allowing visual assessment of the repaired injury components (Fig. 4). Continuous cyclic loading from 2 to 15 N at a rate of 5 N/s for a total of 500 cycles simulated immediate postoperative mobilisation. Upon completion of cyclic loading, specimens were loaded to failure at a rate of 20 mm/min. A high-resolution camera placed in front of the experimental setup documented bony and tendinous gap formation at the repair site at the initial 2 N preload, periodically every 100 cycles, as well as, when loaded to failure, disclosing failure mechanisms. Consistent with methods used in our former studies^{30 (link),31 (link)} on FDP tendon avulsion repair, the load at first noteworthy displacement (2 mm) representing the load at the occurrence of an avulsion fragment- or reattached tendon derived, clinically relevant global system displacement was evaluated. To allow a thorough perception of the repair- and failure biomechanics, following data were recorded and analysed as well: load at failure, elongation of the system, gap formation at the bone–bone contact line (proximalisation of the fragment resulting in an articular step off), gap formation at the bone–tendon insertion line, and the mechanism of failure. Reaching a structural balance of the tendon and the suture material after 50 cycles, elongation of the system was measured between the 50th and the 500th cycle. Bony or tendinous dynamics at the repair site documented on photographs were analysed with Image J Software (National Institutes of Health, Washington, DC, United States).Figure 4

(A) Specimen preparation and injury simulation prior to repair. (B) Repaired specimen secured in the DP holding cylinder mounted on the tensile testing machine. Distance (r) indicating the 10 mm reference for further image evaluation.