Acute Achilles tendon ruptures are frequent injuries, with an annual incidence of 10 to 14 per 100,000.1–3 Middle-aged recreational athletes are affected most frequently.2 In the past, operative and nonoperative treatment normally included the use of orthotic or other stabilization of the injured Achilles tendon (eg, plaster). Clinical data showed increased posttraumatic lengthening with functional loss in conservatively treated patients,4,5 with increased rerupture rates compared with surgically treated patients (7%–20% vs 0.6%–3%, respectively).6–9 In contrast, surgically treated patients exhibited increased infection rates and wound healing problems.6,9–11
Many studies have reported positive effects of early weight bearing on tendon healing in percutaneous Achilles tendon repair.12–19 However, controversy remains regarding whether open repair or nonoperative therapy is best for patients undergoing an early mobilization treatment regimen. A previous animal study showed indifferent functional results in operatively and nonoperatively treated rats; however, only the healing process within the first 2 weeks was examined.20 Comprehensive testing during the early and advanced phases of healing was not performed.
The goal of the current study was to investigate the histological and biomechanical characteristics of early mobilization and full weight-bearing treatment in an Achilles tendon rupture model. The authors hypothesized that operatively treated Achilles tendons would show superior biomechanical properties compared to those treated nonoperatively.
Materials and Methods
This study was approved by the institutional animal care board. All experiments were performed in accordance with the requirements of the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. This animal model is widely used in research.20–22
Eighty adult male Spraque-Dawley rats (400–425 g) were studied. The animals were randomized into 2 groups of 40. At baseline, the right hindpaw Achilles and plantaris tendons of all animals in both groups were dissected. Animals in the operative group underwent Achilles tendon repair using a modified Kessler-type suture. Ten rats in both groups were sacrificed at 1, 2, 4, and 8 weeks following tenotomy. Three sacrificed animals were randomly chosen for histological examination, and specimens from 7 sacrificed rats were used for biomechanical testing. The muscle-Achilles tendon-bone unit was harvested by transecting the middle of the gastrocnemius muscle and the calcaneus.
Animals were anesthetized with isoflurane in a small animal anesthesia machine. An injection of 20 mg/kg of cefazolin and 0.06 mg/kg of buprenorphine was administered intramuscularly into the left thigh for analgesia and antibiotic prophylaxis. The right hindpaw then was shaved and disinfected 3 times using povidone-iodine and rinsed with 70% alcohol.
In an aseptic manner, the rats were laid on a heated surgery table and a sterile drape was placed, leaving only the limb exposed. A 3-cm skin incision was made longitudinally dorsal to the right Achilles tendon, and the peritendon was split. The Achilles tendon was dissected orthogonal to the visible fibers 5 mm proximal to the calcanear insertion. The plantaris tendon was cut to prevent an internal splint phenomenon.20,22,23 The tendons of animals in the nonoperative group were left unsutured, and animals in the operative underwent Achilles tendon repair using a locking technique (Kessler-type suture, PDS II Ethicon 2-0). The wound was closed in layers in both groups.
A heating pad was used to regain normal body temperature. The cefazolinbuprenorphine injection was repeated twice daily for 3 days. Animals were monitored regularly for signs of pain (eg, lack of locomotion, vocal distress, and tenderness) and infection. Rat chow and water were fed ad libitum. No cast was applied to animals in either group, allowing free range of motion as described by Murrell et al.24
Muscle–tendon–bone units were wrapped in cotton gauze soaked with Ringer’s lactated solution and stored at −20°C until testing. Before testing, specimens were put in Ringer’s lactated solution for 4 hours for unfreezing. Tendon thickness at the site of the former transection was measured by fixing the muscle–tendon–bone unit by freezing the muscular segment between the cryo-jaws and the bony segment between the copper clamp.25 Tendon length was determined at the starting point, with the tendon tensioned with 0.1 N, by measuring the distance between the cryo-jaws (gastrocnemius) and copper clamp (calcaneus). The clamp was attached to an electrohydraulic materials testing machine (Zwick GmbH & Co. KG, Ulm-Einsingen, Germany). Before testing the tensile strength, the tendons were not stretched or preconditioned. Temperature was maintained at 25°C (77°F) during the entire procedure. Dehydration of the muscle–tendon–bone units was prevented by keeping them covered with Ringer’s lactated solution when dehydration was observed.
After filling the liquid nitrogen reservoirs, the experiment began as soon as the expansion of the freezing zone reached the border of the metal clamp but did not extend into the tendon substance or repair site (registered manually with a metal needle). The displacement rate was set at 1000 mm/min. Force displacement curves were recorded and transferred to a computer for data analysis. Load to failure (N, peak of the curve) and stiffness (N/mm) were measured.
At 1, 2, 4, and 8 weeks after baseline, 3 specimens of each group were used for histological examination. For dehydration, the specimens were fixated in 4% buffered (pH, 7.4) formalin for 24 hours and embedded in paraffin wax. Sections of 5 μm were taken longitudinally from the midsubstance and stained with hematoxylin-eosin stain. Histological analysis was performed by the semiquantitative score of Bonar, as published by Maffulli et al.26 Collagen ordering, cell number, and tenocyte appearance were evaluated by 2 blinded investigators (D.K., M.M.).
Group comparisons were performed with the nonparametric Mann-Whitney U test. The level of significance concerning a 2-sided alternative hypothesis was set at a P value less than .05.
At 1 and 2 weeks after Achilles tendon dissection, maximum failure load was increased in the operative and nonoperative groups (P<.05) (Figure 1). At 4 and 8 weeks, no significant difference in maximal failure load was observed between the 2 groups.
Figure 1: Effect of operative vs nonoperative treatment on the maximum (max) failure load of rat Achilles tendon over time after transection. The boxes extend from the lower to the upper quartile values of the data, with a line at the median. The whiskers extending from the box show the range, and the circles assign average values of the data set. Upper X-axis tick labels indicate minimal statistical significance (P values) for diverseness of the cohorts.
In the first week, tendon stiffness was significantly increased (P<.01) in the operative group and remained increased in the second week (P<.05). No difference was found at 4 and 8 weeks following tenotomy. Maximum tendon stiffness was found in both groups at 8 weeks following tenotomy (Figure 2).
Figure 2: Effect of operative vs nonoperative treatment on the stiffness of rat Achilles tendon over time after transection. The boxes extend from the lower to the upper quartile values of the data, with a line at the median. The whiskers extending from the box show the range, and the circles assign average values of the data set. Upper X-axis tick labels indicate minimal statistical significance (P values) for diverseness of the cohorts.
Thickness at the tendon repair site was increased (P<.05) in the operative group at 1, 2, and 8 weeks after tenotomy. The callus in the operative group showed a constant diameter of approximately 4 mm, whereas the callus in the nonoperative group varied from 1.7 to 3.6 mm, remodeling down to 3.3 mm at week 8 (Figure 3).
Figure 3: Effect of operative vs nonoperative treatment on the diameter of the repair site of rat Achilles tendon over time after transection. The boxes extend from the lower to the upper quartile values of the data, with a line at the median. The whiskers extending from the box show the range, and the circles assign average values of the data set. Upper X-axis tick labels indicate minimal statistical significance (P values) for diverseness of the cohorts.
The length of the Achilles tendon, measured as the distance between the cryo-jaws (gastrocnemius) and copper clamps (calcaneus), was significantly increased (P<.01) in the nonoperative group throughout the whole observation period (Figure 4).
Figure 4: Effect of operative vs nonoperative treatment on the length of rat Achilles tendon over time after transection. The boxes extend from the lower to the upper quartile values of the data, with a line at the median. The whiskers extending from the box show the range, and the circles assign average values of the data set. Upper X-axis tick labels indicate minimal statistical significance (P values) for diverseness of the cohorts.
Histological examination of the non-operatively treated tendons showed a higher degree of organization and a more homogeneous pattern of collagen fibers at every time point. Collagen formation and arrangement was seen earlier, and maturation of fibroblasts to fibrocytes was faster. At week 8, the appearance of the nonoperatively treated tendon resembled a normal histological pattern of Achilles tendons compared with the left side. Tendon fibers and collagen crimp started to synchronize within big bundles, and fibrocytes were lined up (Figure 5). In contrast, the sutured tendons had fewer organized collagen fibers, and scar formation was seen around the sutures. At week 8, more cells were visible compared with the normal tendon; only partial organized fiber bundles with a lower grade of synchronization could be seen (Figure 6). The mean histological Bonar score showed better results in the nonoperatively treated group (Figure 7).
Figure 5: Effect of nonoperative treatment on the histology of a rat Achilles tendon 8 weeks after transection and suture were performed. The histological pattern is close to a normal Achilles tendon with lined-up fibrocytes (arrows) and big bundles of collagen starting to synchronize (hematoxylin-eosin stain, original magnification ×200).
Figure 6: Effect of operative treatment on the histology of a rat Achilles tendon 8 weeks after transection and suture were performed less organized collagen (arrows) and scar tissue and hypercellularity are visible (hematoxylin-eosin stain, original magnification ×200).
Figure 7: Effect of operative vs nonoperative treatment on the histology of rat Achilles tendons after transection, classified using the Bonar scoring system.
In the current study, histological and biomechanical characteristics of early mobilization and full weight-bearing treatment in an Achilles tendon rupture model were assessed. Operatively and nonoperatively treated animals were observed for 8 weeks. Animals in both groups were allowed unlimited mobility. No cast or orthotic fixation was applied. The recent literature reports the effectiveness of early functional therapy compared with cast immobilization after Achilles tendon rupture.5,14,16,17 The key component of modern rehabilitation is early functional therapy that allows foot and ankle movement in the first week after rupture. The improved outcome after early mobilization has ignited debate among orthopedic surgeons as to which approach is best. Operative treatment results in less maximum force reduction, lower rerupture rates, and earlier return to work or sport. Nonoperative treatment exhibits fewer local complications, especially infections, and generally lower costs. However, the final outcome appears to be comparable between operative and nonoperative treatment regimens.1,5,7,9,14,15,21,27,28
In the current study, full weight-bearing mobilization allowed Achilles tendon healing within 8 weeks in animals in both groups. The main finding was that open repair of ruptured Achilles tendons leads to superior biomechanical results compared to nonoperative treatment. Tendons in the operative group showed a significantly thicker and more stable tendon construct compared with those in the non-operative group. Biomechanical results of both groups differed only 1 and 2 weeks following tenotomy. The superior biomechanical properties after open repair only occurred during the early phase of healing. At 4 weeks after tenotomy, animals in both groups showed indifferent results in stiffness and tensile strength. This is surprising because conservatively treated Achilles tendons were expected to show decreased load capacity throughout the entire observation period.
Similar results were reported by Murrell et al.24 They focused on biomechanical, functional, and morphological advantages of operative vs nonoperative treatment. Functional performance was determined by the measurement of hind-paw prints using the Achilles Functional Index. On day 15, the animals were sacrificed, and biochemical and histological evaluations were performed. By day 15, no functional or failure load impairments were observed after operative or nonoperative treatment.24
The second important finding of the current study is that conservative treatment of dissected Achilles tendons led to increased length and decreased diameter at the defect site. This finding is supported by a study by Konerding et al,29 who showed that sutured Achilles tendons were shorter and thicker in appearance, whereas the nonsutured tendons appeared elongated and more slender. Furthermore, nearly identical maximum tensile strengths (median, 245 vs 253.4 N) were reported 3 months after tenotomy in operatively and nonoperatively treated animals. Indifferent maximum tensile strength was found in both groups when compared with the healthy contra-lateral Achilles tendon (86% to 96%). In a third group, an external fixator made of 3 Kirschner wires was applied to prevent early functional mobilization. In this group, dissected tendons reached only 60% of tensile strength compared with the contralateral side.29 In the current study, tendon length was found to be significantly elongated after nonoperative treatment compared with suture repair.
These results underline the importance of early functional therapy after Achilles tendon rupture.29 A third finding of the current study is that positive effects on tissue remodeling were shown by nonoperatively treated rats. Histological examination showed a higher grade of organization of collagen fibers at all time points in this group. Collagen formation following nonoperative therapy was observed to occur earlier, maturation of fibroblasts to fibrocytes was faster, cellularity was lower, and the specimens showed a typical tendon-like appearance compared with more scar-like tissue in the operative group.
After 12 weeks, Thermann et al30,31 found no significant difference between operative and nonoperative groups regarding histological appearance, collagen III content, or tendon thickness. After 1 to 4 weeks, the current authors found that the nonoperatively treated tendon showed a much higher degree of collagen organization and fibroblast maturation, and after 8 weeks, it appeared far more similar to a normal Achilles tendon than the operatively treated Achilles tendons.
The current authors are aware of the difficulty of applying such results to humans. Besides the difference in healing itself, surgery was not performed using a traumatic-degenerative tendon within this study, as is normally done in humans. Furthermore, the current authors did not use postoperative immobilization in a cast or shoe; the rats immobilized themselves due to pain. The effect of freezing tendons on structural integrity and biomechanical characteristics is still disputed.
This study showed that full weight-bearing mobilization allowed Achilles tendon healing within 8 weeks in a rat model. Surgical treatment of dissected rat Achilles tendons showed superior biomechanical characteristics during the first 2 weeks. Nonoperative treatment resulted in superior histological findings but significant lengthening of the tendon in the early and advanced healing phase (weeks 1 to 8).
- Clayton R, Court-Brown C. The epidemiology of musculoskeletal tendinous and ligamentous injuries. Injury. 2008; 39(12):1338–1344. doi:10.1016/j.injury.2008.06.021 [CrossRef]
- Jozsa L, Kvist M, Balint B, et al. The role of recreational sport activity in Achilles tendon rupture: a clinical, pathoanatomical, and sociological study of 292 cases. Am J Sports Med. 1989; 17(3):338–343. doi:10.1177/036354658901700305 [CrossRef]
- Tumilty S. Achilles tendon rupture: rising incidence in New Zealand follows international trends. Physical Therapy Reviews. 2007; 12(1):59–65. doi:10.1179/108331907X174998 [CrossRef]
- Cetti R, Christensen S, Ejsted R, Jensen N, Jorgensen U. Operative versus nonoperative treatment of Achilles tendon rupture. Am J Sports Med. 1993; 21(6):791–799. doi:10.1177/036354659302100606 [CrossRef]
- Metz M. Acute Achilles tendon rupture: minimally invasive surgery versus nonoperative treatment with immediate full weightbearing—a randomized controlled trial. Am J Sports Med. 2008; 36(9):1688–1694. doi:10.1177/0363546508319312 [CrossRef]
- Bhandari M, Guyatt G, Siddiqui F, et al. Treatment of acute Achilles tendon ruptures: a systematic overview and metaanalysis. Clin Orthop. 2002; (400):190–200.
- Pajala A, Kangas J, Ohtonen P, Leppilahti J. Rerupture and deep infection following treatment of total Achilles tendon rupture. J Bone Joint Surg Am. 2002; 84(11):2016–2021.
- Ingvar J, Tägil M, Eneroth M. Nonoperative treatment of Achilles tendon rupture: 196 consecutive patients with a 7% re-rupture rate. Acta Orthop. 2005; 76(4):597–601. doi:10.1080/17453670510041619 [CrossRef]
- Khan RJ, Carey Smith RL. Surgical interventions for treating acute Achilles tendon ruptures. Cochrane Database Syst Rev.2010; (9):CD003674. doi:/10.1002/14651858 [CrossRef] .CD003674.pub4
- Lo I, Kirkley A, Nonweiler B, Kumbhare D. Operative versus nonoperative treatment of acute Achilles tendon ruptures: a quantitative review. Clin J Sports Med. 1997; 7(3):207–211. doi:10.1097/00042752-199707000-00010 [CrossRef]
- Van der Linden-van der Zwaag H, Nelissen R, Sintenie J. Results of surgical versus non-surgical treatment of Achilles tendon rupture. Int Orthop. 2004; 28(6):370–373.
- Fitzgibbons R, Hefferon J, Hill J. Percutaneous Achilles tendon repair. Am J Sports Med. 1993; 21(5):724–727. doi:10.1177/036354659302100516 [CrossRef]
- Majewski M, Rohrbach M, Czaja S, Ochsner P. Avoiding sural nerve injuries during percutaneous Achilles tendon repair. Am J Sports Med. 2006; 34(5):793–798. doi:10.1177/0363546505283266 [CrossRef]
- Majewski M, Schaeren S, Kohlhaas U, Ochsner P. Postoperative rehabilitation after percutaneous Achilles tendon repair: early functional therapy versus cast immobilization. Disabil Rehabil. 2008; 30(20):1726–1732. doi:10.1080/09638280701786831 [CrossRef]
- Metzl J, Ahmad C, Levine W. The ruptured Achilles tendon: operative and non-operative treatment options. Curr Rev Musculoskelet Med. 2008; 1(2):161–164. doi:10.1007/s12178-008-9025-4 [CrossRef]
- Suchak A, Bostick G, Beaupre L, Durand D, Jomha N. The influence of early weight-bearing compared with non-weight-bearing after surgical repair of the Achilles tendon. J Bone Joint Surg Am. 2008; 90(9):1876–1883. doi:10.2106/JBJS.G.01242 [CrossRef]
- Andersson T, Eliasson P, Aspenberg P. Tissue memory in healing tendons: short loading episodes stimulate healing. J Appl Physiol. 2009; 107(2):417–421. doi:10.1152/japplphysiol.00414.2009 [CrossRef]
- Rodeo S, Delos D, Weber A, et al. What’s new in orthopaedic research. J Bone Joint Surg Am. 2010; 92(14):2491–2501. doi:10.2106/JBJS.J.01174 [CrossRef]
- Twaddle B, Poon P. Early motion for Achilles tendon ruptures: is surgery important?Am J Sports Med. 2007; 35(12):2033–2038. doi:10.1177/0363546507307503 [CrossRef]
- Murrell G, Lilly E III, Collins A, Seaber A, Goldner R, Best T. Achilles tendon injuries: a comparison of surgical repair versus no repair in a rat model. Foot Ankle. 1993; 14(7):400–406.
- Majewski M, Betz O, Ochsner P, Liu F, Porter R, Evans C. Ex vivo adenoviral transfer of bone morphogenetic protein 12 (BMP-12) cDNA improves Achilles tendon healing in a rat model. Gene Ther. 2008; 15(16):1139–1146. doi:10.1038/gt.2008.48 [CrossRef]
- Forslund C, Aspenberg P. OP-1 has more effect than mechanical signals in the control of tissue differentiation in healing rat tendons. Acta Orthop. 1998; 69(6):622–626. doi:10.3109/17453679808999268 [CrossRef]
- Bring DKI, Reno C, Renstrom P, Salo P, Hart DA, Ackermann PW. Joint immobilization reduces the expression of sensory neuropeptide receptors and impairs healing after tendon rupture in a rat model. J Orthop Res. 2009; 27(2):274–280. doi:10.1002/jor.20657 [CrossRef]
- Murrell G, Lilly E III, Goldner R, Seaber A, Best T. Effects of immobilization on Achilles tendon healing in a rat model. J Orthop Res. 1994; 12(4):582–591. doi:10.1002/jor.1100120415 [CrossRef]
- Wieloch P, Buchmann G, Roth W, Rickert M. A cryo-jaw designed for in vitro tensile testing of the healing Achilles tendons in rats. J Biomech. 2004; 37(11):1719–1722. doi:10.1016/j.jbiomech.2004.01.027 [CrossRef]
- Maffulli N, Longo UG, Franceschi F, Rabitti C, Denaro V. Movin and Bonar scores assess the same characteristics of tendon histology. Clin Orthop Relat Res. 2008; 466(7):1605–1611. doi:10.1007/s11999-008-0261-0 [CrossRef]
- Hufner T, Brandes D, Thermann H, Richter M, Knobloch K, Krettek C. Long-term results after functional nonoperative treatment of Achilles tendon rupture. Foot Ankle Int. 2006; 27(3):167–171.
- Majewski M, Ochsner P, Liu F, Flueckiger R, Evans C. Accelerated healing of the rat Achilles tendon in response to autologous conditioned serum. Am J Sports Med. 2009; 37(11):2117–2125. doi:10.1177/0363546509348047 [CrossRef]
- Konerding M, Arlt F, Wellmann A, Li V, Li W. Impact of combinatory growth factor application on rabbit Achilles tendon injury with operative versus conservative treatment: a pilot study. Int J Mol Med. 2010; 25(2):217–224.
- Thermann H, Beck A, Holch M, Biewener A, Bosch U, Frerichs O. Functional treatment of acute Achilles tendon rupture: a histological, immunohistological and ultrasonographic analysis of the animal model [in German]. Unfallchirurg. 1999; 102(6):447–457. doi:10.1007/s001130050434 [CrossRef]
- Thermann H, Frerichs O, Biewener A, Krettek C, Schandelmeier P. Functional treatment of acute rupture of the Achilles tendon: an experimental biomechanical study [in German]. Unfallchirurg. 1995; 98(10):507–513.