Our objective was to evaluate 4 configurations of the OrthoCoupler™ to replace the extensor mechanism of the knee in goats. We demonstrated thorough tissue integration without evidence of microencapsulation or compromise of vascularity. In terms of strength and stiffness, the OrthoCoupler™ legs behaved similarly to the control legs in the needled groups, but elongated more at failure. The OrthoCoupler™ legs also behaved similarly to controls in the barbed groups, but required less force for the fiber bundles to pullout of the muscle. Since the desired ingrowth was found in all groups, there is no reason to believe it contributed any less strength to the barbed groups. Rather, we believe that the barb itself did not affect the strength, whereas the knots of the needled version provided some additional strength.
This outcome suggests that the needled version is more feasible than the barbed version for initial clinical applications. Also, while wire staples and sutures are commonly left in smooth muscle, we are unaware of studies on the long-term effect of such wire in functioning skeletal muscle. The study of potential enhancements for reduced exposure (e.g., a less simplistic barb design) may be useful.
This is the first study in which pull-out strength could be measured, since previously the coupling always exceeded muscle strength [21
]. This does not suggest weaker coupling in this model, however. Forces in the present study were 4.3 times those seen in the semitendinosus study, while the device was only 2.7 times the size, and device-to-muscle proportions of the implantation region were similar [22
]. This 60% higher maximum device stress is due to the fact that the semitendinosus itself tore remotely, without device pull-out, whereas the quadriceps usually remained intact until device pullout had begun. The fact that this muscle withstood proportionally higher loads seems to be due to stress-shielding by the femur through the large aponeurosis along the quadriceps. While equivalent strength to unoperated controls was obtained, this finding suggests even higher attachment strengths might be achievable in the quadriceps by increasing the number of fibers deployed in this muscle.
We believe the OrthoCoupler™
has clinical potential. Cords, rods, and cables work in acute trials [6
], but the sutured tendon connections separate after a few days to a few weeks. Inducing neotendon to grow among prosthetic fibers has been done with carbon[7
], but unfavorable tissue reactions decreased mechanical properties over time. Augmenting repair with tissue factors [39
] requires harvesting and preparation of autogenous or allogenic tissues, and has not matched the strength of intact controls.
In a previous study we tested a smaller OrthoCoupler™
in the goat semitendinosus tendon [22
]. The tendon was removed bilaterally in 8 goats. Left sides were reattached with the device, and right sides were reattached using the Krackow stitch with #5 braided polyester sutures. Fatigue strength of the device in vitro
was several times the contractile force of the muscle. In strength testing at necropsy 60 days post-surgery, suture controls pulled out at 120 ± 68 N, whereas all devices were still holding after the muscle tore, remotely, at 298 ± 111 (mean ± SD) (p<0.0003). Muscle tear strength was reached with the fiber-muscle composite still intact.
This study has limitations. First, the study did not vary the time of evaluation after surgery. Future studies are underway to evaluate repairs at longer time points. Second, semi-quantitative and quantitative histological analyses were not performed. Third, histology was performed in specimens after failure testing. The observed fiber-muscle interface might not be a true representation as disruptive mechanical testing would have resulted in slipping of fibers through the muscles. Four, any bone anchor or prosthesis may be adapted simply to the distal loop(s)of an OrthoCoupler™
, and replaced if needed without disrupting the loop(s). However, this study did not investigate further bone-interface possibilities. Five, in clinical repair or replacement of the extensor mechanism, the ability to integrate a salvaged or prosthetic patella with the OrthoCoupler™
could be advantageous, but such investigations were beyond the scope of this study. Six, the attachment strength of the device at the time of surgery might already be sufficient to sustain in vivo loads, but this was not studied. Caprosyn suture (absorbed in 56 days)was used to allow sufficient time for tissue integration and stability. Seven, during needled-version implantation, extensive exposure of the muscle was required, limiting use of the device to situations where such exposure could be achieved. Eight, in vitro mechanical testing is a limited assessment tool. Although the device’s holding strength exceeds passive muscle strength, this does not imply that the device can withstand all possible in vivo muscle forces. Nine, neither the device pre-implantation nor the excised tendon-patella-tendon structure were tested to compare viscoelastic properties, and estimates of fatigue strength (1867 N) and ultimate strength (4741 N) for the device alone were extrapolated from smaller versions [22
], which do not have a braided strap.
In the current study, in vivo observations, mechanical testing, and histology support adequacy of the repair. The implanted fibers equaled <2% of the muscle cross section in the implanted region. Animals resumed walking with the operated leg within hours. Based on these results, the configuration selected for ongoing, extended-healing studies was needled coated bundles due to easier implantation and mechanical strength similar to the unoperated leg. Histology of fully vascularized integrated tissue encourages us to expect sustained strength in longer-term studies (now underway) and in different tissue models. We believe this technology may be of value for clinical challenges in orthopaedic oncology, an expanded application of tendon-transfer, revision arthroplasty, and tendon injury reconstruction.