Mechanical Behavior of a Novel Tendon Repair Device During Cyclic Loading In Vitro: Comparison With Commonly Used 2-Strand and 4-Strand Suture Repair Techniques in Cadaveric Flexor Tendons

ABSTRACT

Satisfactory clinical outcome following flexor tendon repair remains a challenge to the hand surgeon. Advances in surgical technique and accelerated postoperative rehabilitation protocols are responsible for improvements in postoperative motion, but re-rupture rates continue to be problematic. A novel tendon repair technique (Teno Fix System; Ortheon Medical, Winter Park, Fla) was developed that uses embedded soft-tissue anchors and a multifilament wire to provide a high-strength and low-profile repair. This study compared the mechanical profile of the Teno Fix tendon repair system to current 2- and 4-strand 3-0 suture repair techniques during cyclic loading.

Thirty zone II cadaveric flexor digitorum profundus tendon lacerations (index, middle, and ring digits) were randomized to three different tendon repair techniques (3-0 Modified Kessler, 3-0 locked Cruciate 4-strand, or Teno Fix) performed in situ by a single surgeon. Mechanical behavior of each repair was studied under cyclic load in an attempt to simulate in vivo forces during active motion. The specimens were excised and initially cycled in tension, under load control, between 5 N and 25 N for 8000 cycles at 2 Hz. The peak load increased by 10 N and specimen cycled for an additional 4000 cycles. This 10 N increase was repeated every 4000 cycles to 65 N for a total of 24,000 cycles. If the specimen did not fail by final load step of 65 N and 24,000 cycles, it was loaded in displacement control, at a rate of .2 mm/second until failure. Displacement at the repair site was monitored using a Differential Variable Reluctance Transducer gage (Microstrain, Burlington, Vt), and gap formation was recorded using serial digital photography. Failure was defined as repair rupture or 4-mm gap displacement. Cycles to failure were recorded and weighted by the load step at which the specimen was cycling at failure (N-cycles).

The 3-0 locked cruciate 4-strand repair demonstrated a significant increase in N-cycles to failure when compared with either of the other two suture repairs (P=.018). The cruciate repair exhibited increased resistance to 1-mm gap (16,200 cycles) compared with the Kessler (7200 cycles) or the Teno Fix (6000 cycles) (P=.003). Differential Variable Reluctance Transducer analysis demonstrated less displacement of the cruciate repair at 35 N and 45 N load steps when compared with the two other repairs (P<.01).

Inadequate initial repair strength leads to separation of flexor tendon repairs under the stress of cyclic loading, thus increasing the chance for failure. The Teno Fix implant previously demonstrated promising laboratory and clinical results in range of motion and decreased rupture rates, and may lead to improvements over conventional suture techniques. In this high force cyclical loading protocol, however, neither the Teno Fix nor the Kessler repair conferred mechanical advantages over a 3-0 locked cruciate 4-strand suture repair. The cyclic forces in this testing protocol exceed load demands of unresisted active motion protocols, but the information provided by protocol establishes safe boundaries for active motion rehabilitation of the Teno Fix device.