BACKGROUND: Loss of finger motion after zone II flexor tendon repair remains common and is largely caused by adhesion formation. Although intrinsic vascular and cellular contributions to tendon healing have been examined in canine, avian, and primate models, prior studies have emphasized biochemical or perfusion-based endpoints without comprehensive assessment of functional and mechanical outcomes after clinically relevant repair. Consequently, the influence of intrinsic healing elements on early postoperative function remains unclear. In this study, we employed a novel avian model of intrasynovial (zone II) tendon injury, which enables clinically relevant operative repair, postoperative rehabilitation, and quantitative assessment of tendon function and biomechanics, to investigate the contribution of intrinsic healing elements to tendon repair.
QUESTIONS/PURPOSES: (1) Does operative ablation of intrinsic healing elements worsen early ROM after zone II flexor tendon repair? (2) Does disruption of intrinsic pathways increase adhesion formation, as measured by normalized work of flexion and gross adhesion scores? (3) Does intrinsic tendon component ablation compromise mechanical properties, including ultimate load to failure and stiffness? (4) Does removal of intrinsic elements alter the expression of fibrotic and matrix-related genes during early healing?
METHODS: Using a clinically relevant, large-animal avian (Meleagris gallopavo) model of zone II flexor tendon repair, flexor digitorum longus (FDL) tendons underwent sharp transection and primary repair and were assigned to one of four groups: (1) control repair with intact intrinsic elements (3 weeks postinjury, n = 11; 6 weeks postinjury, n = 18), (2) vincular ligation and transection (3 weeks postinjury, n = 12; 6 weeks postinjury, n = 16), (3) tenocyte cryoablation via intraoperative freeze-thaw devitalization (3 weeks postinjury, n = 13; 6 weeks postinjury, n = 13), or (4) complete tendon devitalization (3 weeks postinjury, n = 13; 6 weeks postinjury, n = 16). A total of 112 male and female birds were used for this study. Harvested tendons were evaluated at 3 and 6 weeks postoperatively. Outcomes included interphalangeal ROM, normalized work of flexion, semiquantitative gross adhesion scoring, ultimate load to failure, stiffness, qualitative histologic assessment, qualitative immunofluorescence, and quantitative PCR for fibrotic and matrix-related genes.
RESULTS: Operative ablation of the intrinsic healing elements did not worsen total ROM and/or increase normalized work of flexion. All experimental groups demonstrated reduced total ROM of the operative digit at 3 weeks postoperatively compared with the contralateral nonoperative digit: mean ± SD control repair with intact intrinsic healing elements (50° ± 10° versus 14° ± 6°, mean difference 36.41° [95% confidence interval (CI) 26.87° to 45.94°]; p < 0.001), vincular disruption group (50° ± 10° versus 14° ± 7°, mean difference 35.97° [95% CI 21.44° to 45.51°]; p < 0.001), tenocyte cryoablation group (50° ± 10° versus 15° ± 8°, mean difference 35.27° [95% CI 25.99° to 44.55°]; p < 0.001), and complete tendon devitalization group (50° ± 10° versus 11° ± 4°, mean difference 38.97° [95% CI 29.44° to 48.50°]; p < 0.001). However, no differences between the groups were detected. Similarly, the total ROM was impaired at 6 weeks postinjury compared with the nonoperative digit in all groups. We did not detect any differences between the groups, and results did not demonstrate time-dependent differences between the two time points. Furthermore, normalized work of flexion, a proxy for peritendinous adhesion severity, was increased in all experimental groups at 3 weeks postinjury: mean ± SD control repair with intact intrinsic healing elements (0 ± 0 N mm/° versus 4 ± 3 N mm/°, mean difference -3.33 [95% CI -5.73 to -0.93]; p = 0.003), vincular disruption group (0 ± 0 N mm/° versus 4 ± 2 N mm/°, mean difference -3.09 [95% CI -5.49 to -0.69]; p = 0.006), tenocyte cryoablation group (0 ± 0 N mm/° versus 3 ± 2 N mm/°, mean difference -2.99 [95% CI -5.33 to -0.66]; p = 0.006), and complete tendon devitalization group (0 ± 0 N mm/° versus 4 ± 2 N mm/°, mean difference -3.31 [95% CI -5.70 to -0.91]; p = 0.003). However, no group differences were detected. At the 6-week follow-up, the normalized work of flexion was also elevated in all groups without any differences between the treatments. Furthermore, the integrity of the tendon intrinsic components has been presumed to be a major determinant of flexor tendon healing strength after zone II tendon repair. However, our results do not support this assumption. At both time points, there were no differences in ultimate load and stiffness between the experimental groups, and there were no group × time interaction effects. Additionally, structural removal of the intrinsic healing elements did not result in meaningful qualitative histologic differences in extrinsic fibrosis, neovascularization, or collagen fiber organization. The profibrotic gene, transforming growth factor beta induced (TGFBI), was upregulated in all experimental groups compared with the normal FDL tendon. Matrix remodeling, measured by matrix metalloproteinase 7 (MMP7) activity, was also activated in all groups relative to the normal FDL tendon. However, no differences were detected between the groups.
CONCLUSION: Operative ablation of intrinsic healing elements, including the flexor vincular system and resident tenocytes, did not worsen early ROM, adhesion formation, or mechanical properties after zone II flexor tendon repair in a clinically relevant large-animal model. Early healing was characterized by a robust fibrovascular response across all groups, indicating that extrinsic mechanisms predominate during the early reparative phase. Paradigms that modulate the extrinsic tendon healing mechanisms could represent a more viable approach to flexor tendon healing outcomes than focusing on bolstering the intrinsic components.
CLINICAL RELEVANCE: Our findings suggest that early functional recovery in this model may depend more on modulation of the extrinsic fibrotic response than on preservation of intrinsic vascular or cellular elements, supporting the development of antiadhesion therapies. Additional preclinical studies are needed to identify the cellular niches that drive extrinsic fibrosis and adhesion formation, determine whether they can be selectively modulated without impairing tendon healing, and establish the translational relevance of these mechanisms across species before clinical application.
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