Skip to main content

Tendon grafts with preserved muscle demonstrate similar biomechanical properties to tendon grafts stripped of muscular attachments: a biomechanical evaluation in a porcine model



(1) To evaluate the biomechanical properties of a porcine flexor digitorum superficialis tendon graft with preserved muscle fibers and (2) to compare these results with the biomechanical properties of a porcine tendon graft after removal of associated muscle.


Eighty-two porcine forelegs were dissected and the flexor digitorum superficialis muscle tendons were harvested. The study comprised of two groups: Group 1 (G1), harvested tendon with preserved muscle tissue; and Group 2 (G2), harvested contralateral tendon with removal of all muscle tissue. Tests in both groups were conducted using an electro-mechanical material testing machine (Instron, model 23-5S, Instron Corp., Canton, MA, USA) with a 500 N force transducer. Yield load, stiffness, and maximum load were evaluated and compared between groups.


The behavior of the autografts during the tests followed the same stretching, deformation, and failure patterns as those observed in human autografts subjected to axial strain. There were no significant differences in the comparison between groups for ultimate load to failure (p = 0.105), stiffness (p = 0.097), and energy (p = 0.761).


In this porcine model biomechanical study, using autograft tendon with preserved muscle showed no statistically significant differences for yield load, stiffness, or maximum load compared to autograft tendon without preserved muscle. The preservation of muscle on the autograft tendon did not compromise the mechanical properties of the autograft.

Level of evidence

Level III Controlled laboratory study


Semitendinosus (ST) and gracilis (G) tendons have been increasingly used as grafts for numerous tendon and ligament structures because of their anatomical features, including a straightforward harvesting technique [6, 8, 11, 25, 27]. These tendons are also commonly used for anterior cruciate ligament (ACL) reconstruction, elbow ligament reconstruction, medial patellofemoral ligament reconstruction, and many other injuries [1, 22, 23, 26, 30,31,32].

When used in ACL reconstruction, hamstring autografts undergo a ligamentization process [15]. This remodeling process involves recellularization, revascularization, changes in the collagen structure, decreased density of type I collagen fibers, and increased type III collagen ( which has lower mechanical strength than type I collagen) [2, 3, 13, 24]. During the early stages of healing, the graft is not well incorporated into the bone and may be susceptible to retear when excessive loads are applied to the knee [18, 19]. Autograft tendons are commonly stripped of the adjacent muscular tissue before graft preparation [29]. This process inevitably causes a certain degree of trauma to the tendon and, on a cellular level, may compromise graft healing [17]. Funchal et al., described the clinical and histological advantages of preserving the adherent muscle tissue during ACL reconstruction, with improved knee function scores, return to sport, and increased final size of the autograft used [9].

Thus, the aim of this study was (1) to evaluate the biomechanical properties of a porcine flexor digitorum superficialis tendon graft with preserved muscle fibers and (2) to compare these results with the biomechanical properties of a porcine tendon graft after removal of associated muscle. We hypothesized that the preservation of muscle fibers adhered to the harvested tendon would not make it more susceptible to failure.

Material and methods

A total of eighty-two porcine forelimbs from two MS60 and F1 breeds were evaluated and their flexor digitorum superficialis muscle tendons were harvested for biomechanical analysis. This was a controlled laboratory study with an allocation ratio of 1:1.

The research was approved by the Research Ethics Committee of the institution (Hospital Governador Celso Ramos – 26,821,019.2.0000.5360).

Inclusion criteria were as follows: pigs aged 28 ± 1 weeks, weighing 105 ± 5 kg and without any evident malformation and/or deformities of their limbs. Pigs were then sacrificed by a veterinarian and their forelimbs were dissected bilaterally. In case of anatomical anomaly or failure of flexor tendon removal, the pair of limbs (right and left) were excluded from the study.

Two groups were randomized using sealed envelopes—Group 1: harvested flexor tendon with muscle preservation; and Group 2: harvested contralateral tendon with removal of all muscle tissue (Fig. 1).

Fig. 1
figure 1

Flowchart for the selection of limbs included in the evaluation. A total of 41 tendons were evaluated in each group

Flexor tendon harvest technique

The flexor digitorum superficialis tendons of the two forelimbs were harvested from each of the 41 pigs, totaling 82 flexor tendon autografts. The anatomy of the porcine forelimb flexor digitorum tendon is sufficiently similar to that of the human semitendinosus and gracilis tendons (knee flexors) and makes it a suitable alternative for testing purposes [4, 14, 21]. The harvest followed the same technique as hamstring tendon harvest in humans as shown in Fig. 2 [8].

Fig. 2
figure 2

A shows the anatomical comparison between the human semitendinosus and gracilis tendons (knee flexors) (left), and the porcine flexor digitorum superficialis (right). The two musculotendinous complexes are quite similar in function and assist with the flexion of the leg at knee. In the biomechanical evaluation of the porcine flexor digitorum superficialis muscle tendon, with and without muscle (B), we attempted to identify the most similar structure with the flexor tendons used for ACL reconstruction

The flexor digitorum superficialis muscle tendons were identified. Next, the tendons were harvested using tendon strippers (Smith & Nephew/Acufex Slotted Tendon Stripper®), the same instruments used for the harvesting of human hamstring tendons. If the tendon of a limb was selected for G1, all muscular attachments were maintained. Conversely, the tendon of the contralateral limb of the same animal was included in G2, and all muscles adjacent to the tendon were removed (Fig. 3).

Fig. 3
figure 3

The three stages of dissection of porcine flexor digitorum superficialis tendon. A shows the right and left limbs before dissection. B and C exhibits the pes anserinus after dissection of the superficial layers of the leg. D shows a standard tendon stripper (Smith & Nephew/Acufex Slotted Tendon Stripper), and E, the details of the open-end of the graft harvester, which is used to release the tendon proximately without distal disinsertion

Biomechanical test

The mounting and the mechanical testing were performed on a single limb of graft, using only the portion of tendon with muscle tissue attached in the G1 group (Fig. 4A-B). Fixation of the tendon ends were achieved using the cryoclamps of an Instron TestMaster Automation System, maintaining a 30 mm length. A cryoclamp (a thermally-based soft tissue clamp) was used to avoid soft tissue slippage or rupture at the clamp-tendon interface. These devices are capable of rapid cooling to freeze the specimen and significantly increase the hold force that can be applied without damaging the tissue as shown in Fig. 4C and 4D.

Fig. 4
figure 4

After harvesting the porcine flexor digitorum superficialis muscle tendon (A), the musculotendinous tissue was cut for comparison of both tendon tissues under the same conditions (B). Each pair of tendons was tested in an Instron TestMaster Automation System: the tendon with muscle (C) and the tendon without muscle (D). The distance between the tendon fixation points was 30 mm. The cryoclamp (pink liquid inside the hoses) was used to reduce the risk of slippage of the tested structure

Traction and overload tests were conducted in both groups using an electro-mechanical material testing machine (Instron, model 23-5S, Instron Corp., Canton, MA, USA) with a 500 N force transducer. The tensile testing was carried out under displacement control. The test started with a crosshead velocity of 10 mm/min until reaching 10 N preload. The displacement value was then reset, the crosshead extension rate changed to 20 mm/min, and the tendons were pulled until failure. The variables analyzed and compared in the biomechanical test were as follows: yield load, stiffness, and maximum load.

Statistical analysis

To evaluate the effect of the muscle stripping process on strength, stiffness, and energy, a two-tailed paired t test with a confidence level of 95% (α = 0.05) was used. An a priori power analysis included the variables strength, resistance, and energy as observed in previous studies and was based on the primary hypothesis that graft harvesting has been associated with a decrease in tissue resistance. It was determined that a sample size of 27 tendons per group would detect a 10% change in resistance index with 80% power and 5% significance.


The results obtained in the present study showed that there was no difference between groups in relation to gender, weight, and age of the porcine donor (Table 1).

Table 1 Demographic data of pigs

The behavior of the porcine grafts during the tests followed the same pattern of stretching, deformation, and failure observed in human grafts subjected to axial strain. There were no significant differences in the comparison between groups for yield load (p = 0.105), stiffness (p = 0.097), and maximum load (p = 0.761), as outlined in Figs. 5 and 6.

Fig. 5
figure 5

Comparison between the mean force applied in G1 (muscle tissue preservation) and G2 (removal of muscle tissue) for the variables yield load (A), tendon stiffness (B), and energy up to maximum load (C). For all the comparisons, there was no significant difference between groups (p = 0.11; 0.1; 0.76, respectively)

Fig. 6
figure 6

(A) shows comparative images of the tendons in G1 (above) and G2 (below): phase (a) represents the relaxed state of the tendon; phase (b) shows the moment of maximum force sustained by the tendons; phase (c) shows the rupture of the first fibers; and phase (d), tendon failure. (B) illustrates the tensile strength and the deformation of the tendons


The most important finding of the present study was that the preservation of muscle tissue in the harvested porcine flexor digitorum superficialis tendon did not have negative effects on its biomechanical performance when compared to tendon that had its musculature removed. This is in contrast to Sun et al., which concluded that excessive amounts of retained skeletal muscle weaken tendon graft's strength in the setting of ACL reconstruction [28].

In a clinical study, Funchal et al. reported that the use of the knee flexor tendon autograft with the preservation of adjacent muscle tissue demonstrated biological and regenerative potential in patients who underwent ACLR [9]. They showed increased graft size as well as favorable Tegner activity scale and Lysholm scores a minimum of two years postoperatively. Further, the study demonstrated a favorable histological characterization during the healing process. Ćuti et al., presented data supporting the capacity of muscle-derived cells to differentiate into tendon tissue [5], and other studies have also demonstrated how tendon muscle remnants improved ACL graft healing [10] Sun et al. also concluded that muscle left on tendon autografts promoted intra-articular healing and remodeling of the graft [28].

Given the established volumetric increase in final graft diameter with muscle preservation, the need for a biomechanical evaluation of the tendon structure with or without the preservation of muscle tissue was necessary [9]. In the present study, the maximum load, stiffness, and the yield load required for tendon failure were evaluated in a porcine model. The preservation of muscle tissue on the grafts tested in this study did not compromise its biomechanical performance. This demonstrates that, in addition to the cellular level advantages, graft muscle preservation does not compromise biomechanical properties when compared to a purely tendon autograft.

Although the biomechanical testing was performed in a porcine model, the flexor digitorum superficialis was used which has comparable anatomy to the hamstring tendons in the human knee. Previous studies have already shown that the porcine flexor digitorum superficialis tendons are considered comparable to human hamstring tendons in relation to the anatomical and histological characteristics [12, 16]. Besides, according to Woo et al., their biomechanical properties demonstrate a rate of deformity similar to lower limbs human tendons [33].

Thus, this biomechanical analysis of the musculotendinous tissue of a pig represents what could be expected in human tissue. The present study also demonstrated that the porcine tendon properties displayed during testing were similar to the pattern of stretching, deformation, and failure observed in human tendons under the effect of axial strain [7, 20].

The effect of removal of muscle tissue from tendon grafts on tendon strength was previously evaluated by Sun et al. Their results were similar to that of the present study, despite the use of different methodology [28]. The time zero biomechanical properties of the grafts are only part of the equation, and there are deleterious histological effects on tendon grafts subjected to muscle stripping.

The weakening of the graft caused by muscle removal starts during surgery at the time of graft preparation and may have deleterious effects on the "ligamentization" process and ligament remodeling. Okazaki et al., demonstrated that muscle stripping of the hamstring tendons caused histological alterations and damage to type I collagen. The density of type I collagen fibers decreased with increasing number of strips necessary to remove musculature [17].

The biological benefits and robust biomechanical profile of grafts with preserved muscle attachments suggest that their use is a reasonable practice especially given the traumatic effect of muscle stripping on a graft’s cellular properties.

Study limitations

The present study has some limitations. First, porcine tendons were used instead of human tendons. This choice, however, was made due to their immediate availability and low cost. Second, the single strand tendon used did not represent the graft diameter in a real clinical situation. Finally, the time zero nature of biomechanical studies was unable to evaluate the potential of improved healing in grafts with preserved muscle. Improved healing of grafts with preserved muscle may be clinically relevant as a way to improve the healing of ACL reconstruction in humans.


In this porcine model biomechanical study, using autograft tendon with preserved muscle showed no statistically significant differences for yield load, stiffness, or maximum load compared to autograft tendon without preserved muscle. The preservation of muscle on the autograft tendon did not compromise the mechanical properties of the autograft.


  1. Abyar E, Cone B, McKissack H, Johnson M (2020) A novel technique in treatment of calcaneocuboid dislocation with bifurcate ligament reconstruction using a semitendinosus allograft: a case report. J Bone Joint Surg Case Connect 10(1):e0205

    Article  Google Scholar 

  2. Amiel D, Frank C, Harwood F, Fronek J, Akeson W (1984) Tendons and ligaments: a morphological and biochemical comparison. J Orthop Res 1(3):257–265

    Article  CAS  Google Scholar 

  3. Bosch U, Kasperczyk WJ, Oestern HJ, Tscherne H (1994) The patellar tendon graft for PCL reconstruction. Morphological aspects in a sheep model. Acta Orthop Belg 60 Suppl 1:57–61

    CAS  PubMed  Google Scholar 

  4. Cone SG, Warren PB, Fisher MB (2017) Rise of the pigs: utilization of the porcine model to study musculoskeletal biomechanics and tissue engineering during skeletal growth. Tissue Eng Part C Methods 23(11):763–780

    Article  Google Scholar 

  5. Ćuti T, Antunović M, Marijanović I, Ivković A, Vukasović A, Matić I, Pećina M, Hudetz D (2017) Capacity of muscle derived stem cells and pericytes to promote tendon graft integration and ligamentization following anterior cruciate ligament reconstruction. Int Orthop 41(6):1189–1198 (Epub 2017. Erratum in: Int Orthop.41(6):1287)

    Article  Google Scholar 

  6. Dheerendra SK, Khan WS, Singhal R, Shivarathre DG, Pydisetty R, Johnstone D (2012) Anterior cruciate ligament graft choices: a review of current concepts. Open Orthop J 6:281–286

    Article  Google Scholar 

  7. Domnick C, Herbort M, Raschke MJ, Schliemann B, Siebold R, Śmigielski R, Fink C (2017) Converting round tendons to flat tendon constructs: Does the preparation process have an influence on the structural properties? Knee Surg Sports Traumatol Arthrosc 25(5):1561–1567

    Article  CAS  Google Scholar 

  8. Ferretti A, Conteduca F, Morelli F, Masi V (2002) Regeneration of the semitendinosus tendon after its use in anterior cruciate ligament reconstruction: a histologic study of three cases. Am J Sports Med 30(2):204–207

    Article  Google Scholar 

  9. Funchal LFZ, Ortiz R, Jimenez A, Funchal GDG, Cohen M, Astur DC. (2021) Remnant muscle preservation on hamstring tendon autograft during ACL reconstruction promotes volumetric increase with biological and regenerative potential. Orthopaedic J Sports Med 10;9(3):2325967121990016.

  10. Ghebes CA, Groen N, Cheuk YC, Fu SC, Fernandes HM, Saris DBF (2018) Muscle-secreted factors improve anterior cruciate ligament graft healing: an in vitro and in vivo analysis. Tissue Eng - Part A 24(3–4):322–334

    Article  CAS  Google Scholar 

  11. Hamner DL, Brown CH Jr, Steiner ME, Hecker AT, Hayes WC (1999) Hamstring tendon grafts for reconstruction of the anterior cruciate ligament: biomechanical evaluation of the use of multiple strands and tensioning techniques. J Bone Joint Surg Am 81(4):549–557

    Article  CAS  Google Scholar 

  12. Havulinna J, Leppänen OV, Järvinen TL, Göransson H (2011) Comparison of modified Kessler tendon suture at different levels in the human flexor digitorum profundus tendon and porcine flexors and porcine extensors: an experimental biomechanical study. J Hand Surg [Br] 36(8):670–676

    Article  CAS  Google Scholar 

  13. Liu SH, Yang RS, Al-Shaikh R, Lane JM. (1995) Collagen in tendon, ligament, and bone healing: a current review. Clin Orthop Relat Res (318):265–78. PMID: 7671527

  14. Martin RK, Gillis D, Leiter J, Shantz JS, MacDonald P (2016) A Porcine knee model is valid for use in the evaluation of arthroscopic skills: a pilot study. Clin Orthop 474(4):965–970

    Article  Google Scholar 

  15. Mayr HO, Stoehr A, Dietrich M, von Eisenhart-Rothe R, Hube R, Senger S, Suedkamp NP, Bernstein A (2012) Graft-dependent differences in the ligamentization process of anterior cruciate ligament grafts in a sheep trial. Knee Surg Sports Traumatol Arthrosc 20(5):947–956

    Article  Google Scholar 

  16. Omar M, Dratzidis A, Klintschar M, Kwisda S, Krettek C, Ettinger M (2016) Are porcine flexor digitorum profundus tendons suitable graft substitutes for human hamstring tendons in biomechanical in vitro-studies? Arch Orthop Trauma Surg 136(5):681–686

    Article  Google Scholar 

  17. Okazaki Y, Furumatsu T, Maehara A, Miyazawa S, Kamatsuki Y, Hino T, Ozaki T (2019) Histological alterations to the hamstring tendon caused by cleaning during autograft preparation. Muscles Ligaments Tendons J 9(2):217–224

    Article  Google Scholar 

  18. Papageorgiou CD, Ma CB, Abramowitch SD, Clineff TD, Woo SLY (2001) A multidisciplinary study of the healing of an intraarticular anterior cruciate ligament graft in a goat model. Am J Sports Med 29(5):620–626

    Article  CAS  Google Scholar 

  19. Park SY, Oh H, Park S, Lee JH, Lee SH, Yoon KH (2013) Factors predicting hamstring tendon autograft diameters and resulting failure rates after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 21(5):1111–1118

    Article  Google Scholar 

  20. Piedade SR, Dal Fabbro IM, Mischan MM (2005) Graft semitendinosus and gracilis human muscle tendons elongation: a study carried out on young adult human cadavers. Acta Ortop Bras 13(1):28–30

    Article  Google Scholar 

  21. Proffen BL, McElfresh M, Fleming BC, Murray MM (2012) A comparative anatomical study of the human knee and six animal species. Knee 19(4):493–499

    Article  Google Scholar 

  22. Quach T, Jazayeri R, Sherman OH, Rosen JE (2010) Distal biceps tendon injuries–current treatment options. Bull NYU Hosp Jt Dis 68(2):103–111

    PubMed  Google Scholar 

  23. Ridley TJ, Macalena JA, Arendt EA (2018) Isolated medial patellofemoral ligament reconstruction with semitendinosus tendon allograft. J Bone Joint Surg Essent Surg Tech 8(1):e5

    Article  Google Scholar 

  24. Scheffler SU, Unterhauser FN, Weiler A (2008) Graft remodeling and ligamentization after cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 16(9):834–842

    Article  CAS  Google Scholar 

  25. Shaerf DA, Pastides PS, Sarraf KM, Willis-Owen CA (2014) Anterior cruciate ligament reconstruction best practice: a review of graft choice. World J Orthop 5(1):23–29

    Article  Google Scholar 

  26. Singer G, Ferlic P, Kraus T, Eberl R (2013) Reconstruction of the sternoclavicular joint in active patients with the figure-of-eight technique using hamstrings. J Shoulder Elbow Surg 22(1):64–69

    Article  Google Scholar 

  27. Stevanović V, Blagojević Z, Petković A, Glišić M, Sopta J, Nikolić V, Milisavljević M (2013) Semitendinosus tendon regeneration after anterior cruciate ligament reconstruction: can we use it twice? Int Orthop 37(12):2475–2481

    Article  Google Scholar 

  28. Sun L, Hou C, Wu B, Tian M, Zhou X (2013) Effect of muscle preserved on tendon graft on intra-articular healing in anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 21(8):1862–1868

    Article  Google Scholar 

  29. Vinagre G, Kennedy NI, Chahla J, Cinque ME, Hussain ZB, Olesen ML, LaPrade RF (2017) Hamstring graft preparation techniques for anterior cruciate ligament reconstruction. Arthrosc Tech 6(6):e2079–e2084

    Article  Google Scholar 

  30. West RV, Harner CD (2005) Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg 13:197–207

    Article  Google Scholar 

  31. Widner M, Dunleavy M, Lynch S (2019) Outcomes following ACL reconstruction based on graft type: are all grafts equivalent? Curr Rev Musculoskelet Med 12(4):460–465

    Article  Google Scholar 

  32. Wilson WK, Morris R, Coskey A, Smith B, Gugala Z (2019) Quadriceps augmentation of undersized hamstrings during ACL reconstruction. Knee 26(1):73–78

    Article  Google Scholar 

  33. Woo SL-Y, Ritter MA, AMiel D, Sanders TM, Gomez MA, Kei SC, Akeson WH. (1980) The biomechanical and biochemical properties of swine tendons - long term effects of exercise on the digital extensors. Connect Tissue Res 7(3):177–183

    Article  CAS  Google Scholar 

Download references

Author information

Authors and Affiliations



The author(s) read and approved the final manuscript.

Corresponding author

Correspondence to Diego Costa Astur.

Ethics declarations

Competing interests

All authors declare no potential conflict of interest or compliance.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Funchal, L.F.Z., Astur, D.C., Pizzolatti, A.L.A. et al. Tendon grafts with preserved muscle demonstrate similar biomechanical properties to tendon grafts stripped of muscular attachments: a biomechanical evaluation in a porcine model. J EXP ORTOP 8, 57 (2021).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: