Neovascularization of the flexor digitorum profundus tendon after avulsion injury: an in vivo canine study

Tendon grafts: their natural history, biology and future development

The use of tendon grafts has diminished as regimes of primary repairs and rehabilitation have improved, but they remain important in secondary reconstruction. Relatively little is known about the cellular biology of grafts, and the general perception is that they have little biological activity. The reality is that there is a wealth of cellular and molecular changes occurring with the process of engraftment that affect the quality of the repair. This review highlights the historical perspectives and modern concepts of graft take, reviews the different attachment techniques and revisits the biology of pseudosheath formation. In addition, we discuss some of the future directions in tendon reconstruction by grafting, which include surface modification, vascularized tendon transfer, allografts, biomaterials and cell-based therapies.

Flexor tendon injury, repair and rehabilitation

Injuries to the flexor tendons remain among the most difficult problems in hand surgery. Historically, lacerations to the intrasynovial portion of the flexor tendons were thought to be unsuitable for primary repair. Despite continuing advances in our knowledge of flexor tendon biology, repair, and rehabilitation, good results following primary repair of flexor tendons remain challenging to achieve.

Tendon grafts with peritendinous areolar tissue increase the survival of endotendinous tissues and diminish postoperative shrinkage

BACKGROUND

We observed greater postoperative shrinkage of grafted fascia lata without peritendinous areolar tissue (PAT) than with PAT in blepharoptosis surgery. In this article, we sought to evaluate the influence of PAT on tendon grafts.

METHODS

Pairs of anterior tibial tendons with or without PAT were grafted onto the backs of 12 adult Wistar rats. The cephalic and caudal ends of the tendons with or without PAT were fixed tightly on the fascia of the erector spinae muscles with 6/0 polypropylene sutures. A 7-mm distance was maintained between the sutures. Changes in the length between the sutures as well as changes in the histology of the tendons with or without PAT were evaluated at 1 and 5 months after grafting.

RESULTS

The mean lengths of the grafted tendons with PAT at 1 and 5 months after grafting were significantly longer than those without PAT at 1 and 5 months after grafting, respectively (5.4 mm and 4.5 mm vs. 3.8 mm and 2.9 mm, respectively; Wilcoxon signed-rank test; P < 0.05). The grafted tendons without PAT displayed central necrosis and scar formation, whereas those with PAT displayed no alteration in the tissue properties.

CONCLUSIONS

At both 1 and 5 months after grafting, the grafted tendons with PAT were significantly longer and survived better than those without PAT. Tendon grafts with PAT had increased survival of the endotendinous tissue and diminished postoperative shrinkage, since the vascular network of the PAT potentially contributes to early revascularisation of the graft.

Vascular perfusion of a flexor carpi ulnaris muscle turnover pedicle flap for posterior elbow soft tissue reconstruction: a cadaveric study

PURPOSE

The use of a pedicled flexor carpi ulnaris (FCU) muscle proximal turnover flap has been described previously for soft tissue reconstruction at the posterior elbow. Whereas consistent arterial supply to the FCU has been reported, the reliability of distal flap perfusion has not been confirmed. This study evaluated the vascular perfusion of an FCU turnover flap, based on the most proximal primary vascular pedicle that would permit a proximal turnover flap reconstruction to include the olecranon tip.

METHODS

In 12 fresh-frozen, proximal humeral human amputation specimens, the FCU flap was elevated from distal to proximal, preserving the most proximal primary vascular pedicle to the muscle belly that would permit flap coverage of the olecranon tip. The axillary artery was injected with India ink after ligation of radial and ulnar arteries at the wrist. After injection, each specimen was sectioned transversely at 0.5-cm increments to assess vascular perfusion of the muscle using loupe magnification.

RESULTS

The distance from the olecranon tip to the distal FCU muscle belly was 25.9 cm. The primary vascular pedicle that would facilitate creation of a proximal turnover flap was, on average, 5.9 cm distal to the olecranon tip. Perfusion of FCU muscle as measured distal to this primary pedicle was present in 50% to 100% of the muscle belly at an average of 8.9 cm beyond the pedicle. Perfusion of 25% to 50% of the FCU muscle belly was present at an average of 11.1 cm beyond the pedicle. Perfusion became less consistent (<25%) within the muscle belly at an average distance of 11.6 cm.

CONCLUSIONS

Use of a proximally based, pedicled FCU muscle turnover flap provides a reliable option for soft tissue reconstruction at the posterior elbow. We observed consistent arterial perfusion of the muscle flap when preserving a proximal vascular pedicle 5.9 cm distal to the olecranon tip.

The effect of muscle loading on flexor tendon-to-bone healing in a canine model

Previous tendon and ligament studies have demonstrated a role for mechanical loading in tissue homeostasis and healing. In uninjured musculoskeletal tissues, increased loading leads to an increase in mechanical properties, whereas decreased loading leads to a decrease in mechanical properties. The role of loading on healing tissues is less clear. We studied tendon-to-bone healing in a canine flexor tendon-to-bone injury and repair model. To examine the effect of muscle loading on tendon-to-bone healing, repaired tendons were either cut proximally (unloaded group) to remove all load from the distal phalanx repair site or left intact proximally (loaded group). All paws were casted postoperatively and subjected to daily passive motion rehabilitation. Specimens were tested to determine functional properties, biomechanical properties, repair-site gapping, and bone mineral density. Loading across the repair site led to improved functional and biomechanical properties (e.g., stiffness for the loaded group was 8.2 +/- 3.9 versus 5.1 +/- 2.5 N/mm for the unloaded group). Loading did not affect bone mineral density or gapping. The formation of a gap between the healing tendon and bone correlated with failure properties. Using a clinically relevant model of flexor tendon injury and repair, we found that muscle loading was beneficial to healing. Complete removal of load by proximal transection resulted in tendon-to-bone repairs with less range of motion and lower biomechanical properties compared to repairs in which the muscle-tendon-bone unit was left intact.

Early healing of flexor tendon insertion site injuries: Tunnel repair is mechanically and histologically inferior to surface repair in a canine model

Orthopedic injuries often require surgical reattachment of tendon to bone. Tendon ends can be sutured to bone by direct apposition to the bone surface or by placement within a bone tunnel. Our objective was to compare early healing of a traditional surface versus a novel tunnel method for repair of the flexor digitorum profundus (FDP) tendon insertion site in a canine model. A total of 70 tendon-bone specimens were analyzed 0, 5, 10 or 21 days after injury and repair, using tensile and range of motion mechanical testing, histology and densitometry. Ultimate force (a measure of repair strength) did not differ between surface and tunnel repairs at day 0. Both repair types had reduced strength at 10 and 21 days compared to 0 days, indicative of deterioration of suture grasping strength (tendon softening). At 21 days, tendons repaired in a bone tunnel had 38% lower ultimate force compared to surface repairs (p = 0.017). Histological findings were comparable between repair groups at 5 and 10 days but differed at 21 days, when we saw evidence of maturation of the tendon-bone interface in the surface repairs compared to an immature fibrous interface with no evidence of tendon-bone integration in the tunnel repairs. After accounting for bone removed by the tunnel, no difference in bone mineral density or trabecular bone volume existed between surface and tunnel repairs. If the results of our animal study extend to healing of the human FDP insertion, they indicate that FDP tendons should be reattached to the distal phalanx by suture to the cortical surface rather than suture in a bone tunnel.

Effect of several growth factors on canine flexor tendon fibroblast proliferation and collagen synthesis in vitro

PURPOSE

Growth factor delivery may be useful to accelerate the rate of tendon healing. Before in vivo use, however, the effects of growth factors on tendon cells need to be well characterized. The purpose of this study was to evaluate the effects of 4 growth factors on intrasynovial tendon fibroblast proliferation and collagen production in vitro. Our first hypothesis was that platelet-derived growth factor BB (PDGF-BB) and basic fibroblast growth factor (bFGF) would promote cell proliferation and collagen production. Our second hypothesis was that there would be a positive effect from the combination of PDGF-BB and bFGF.

METHODS

The growth factors PDGF-BB, bFGF, vascular endothelial growth factor (VEGF), and bone morphogenetic protein 2 (BMP-2) were evaluated in vitro with canine flexor tendon fibroblasts. The effects of single factors (PDGF-BB, bFGF, VEGF, or BMP-2) or a combination of factors (PDGF-BB and bFGF) on cell proliferation (ie, thymidine incorporation) and collagen production (ie, proline incorporation) were evaluated.

RESULTS

The results supported our hypotheses. Cell proliferation increased significantly with PDGF-BB and bFGF. Collagen production also increased significantly with PDGF-BB and bFGF. Cell proliferation and collagen production were unchanged with VEGF and BMP-2. A dose-response effect was seen for PDGF-BB combined with bFGF. The combination of PDGF-BB and bFGF led to an increase in cell proliferation but no change in collagen production compared with each factor alone.

CONCLUSIONS

The growth factors PDGF-BB and bFGF significantly increased flexor tendon fibroblast proliferation and matrix synthesis when applied singly. Administration of PDGF-BB and bFGF combined led to increased proliferation to single factors.

Tendon injury response: assessment of biomechanical properties, tissue morphology and viability following flexor digitorum profundus tendon transection

Insertion site injuries of the flexor digitorum profundus (FDP) tendon often present for delayed treatment. Apart from gross observations made at the time of surgery, the changes that occur in the flexor tendon stump during the interval from injury to repair are unknown. These changes may include tendon softening and loss of viability, which may contribute to the poor outcomes observed clinically and experimentally. Thirty-eight FDP tendons from 23 adult dogs were transected sharply from their insertions on the distal phalanges and were not repaired. Dogs were allowed full weight bearing and were euthanized 7 or 21 days after injury. Biomechanical testing indicated that the resistance of injured tendons to pullout of a Kessler-type suture was not different from control tendons at 7 days and was increased at 21 days by 25% (p<0.05). Morphologically, at 7 and 21 days the cut surface had a smooth appearance and the end of the injured tendon was increased in thickness by 30% and 50%, respectively (p<0.05). Histologically, we observed increased cellularity and dramatic fibroblast proliferation within the injured tendon stump; there was no evidence of decreased cell viability. We conclude that during the interval from 0 to 21 days after FDP insertion-site injury, tendons cells are viable, proliferative and synthesizing new matrix. This leads to increased tendon size and enhanced resistance to suture pullout. These findings offer a scientific rationale supporting the clinical practice of surgical re-attachment within the first 3 weeks after injury.