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Journal of Biomechanics 45 (2012) 289–296

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Journal of Biomechanics

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Effects of tendon and muscle belly dissection on muscular force transmission following tendon transfer in the rat

Huub Maas n, Peter A. Huijing

Research Institute MOVE, Faculty of Human Movement Sciences, VU University, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands

article info abstract

Article history: The aim of the present study was to quantify to what extent the scar tissue formation following the Accepted 22 October 2011 transfer of flexor carpi ulnaris (FCU) to the distal tendon of extensor carpi radialis (ECR) affects the force transmission from transferred FCU in the rat. Five weeks after recovery from surgery (tendon transfer Keywords: group) and in a control group, isometric length–force characteristics of FCU were assessed for Skeletal muscle progressive stages of dissection: (i) with minimally disrupted connective tissues, (ii) after full dissection Myofascial force transmission of FCU distal tendon exclusively, and (iii) after additional partial dissection of FCU muscle belly. Tendon transfer Total and passive length–force characteristics of transferred and control FCU changed significantly Scar tissue by progressive stages of dissection. In both groups, tendon dissection decreased passive FCU force Length–force characteristics exerted at the distal tendon, as well as the slope of the length–force curve. However, force and slope Flexor carpi ulnaris changes were more pronounced for transferred FCU compared to controls. No additional changes occurred after muscle belly dissection. In contrast, total force increased in transferred FCU following both tendon and muscle belly dissection at all lengths studied, while dissection decreased total force of control FCU. In addition, after tendon and muscle belly dissection, we found decreased muscle belly lengths at equal muscle–tendon complex lengths of transferred FCU. We conclude that scar tissue limits the force transmission from transferred FCU muscle via the tendon of insertion to the skeleton, but that some myofascial connectivity of the muscle should be classified as physiological. & 2011 Elsevier Ltd. Open access under the Elsevier OA license.

1. Introduction from its extensor to a flexor site of the knee (Asakawa et al., 2002; Huijing, 1999). Both computer simulations and anatomical stu- Tendon transfers are surgical procedures intended to improve dies indicated that RF should have become a knee flexor after that gait or upper extremity function in individuals with severe surgery (Delp et al., 1994). However, one year or more post disabilities. Ideally, transferred muscles convert completely from operation, intramuscularly stimulated RF still generated a knee their previous mechanical effect (e.g., joint flexion and adduction) extension moment (Riewald and Delp, 1997). Abnormal low into moment generators according to their new path and relo- signal intensity in T2-weighted magnetic resonance images near cated insertion (e.g., joint extension and abduction). This criterion RF muscle was interpreted as evidence of scar tissue formation is assumed to be fulfilled in modeling tendon transfers biome- following tendon transfer (Gold et al., 2004). Such tissues are chanically (e.g., Herrmann and Delp, 1999; Holzbaur et al., 2005; likely to hamper force transmission to the new insertion. How- Veeger et al., 2004). However, such models consist of muscles that ever, studies aimed at quantifying effects of scar tissue on are connected to the skeleton exclusively at their origin and mechanical characteristics of transferred muscle after recovery insertion, ignoring effects of epimuscular myofascial force trans- from the surgery are not available. mission (Huijing, 2009; Maas and Sandercock, 2010) and post- Therefore, the aim of the present study was to quantify to operative tissue responses such as scar tissue formation. what extent the scar tissue formation following an agonist-to- Scar tissue, linking the transferred muscle to surrounding antagonist tendon transfer affects force transmission from trans- structures, is considered as a factor that limits the functional ferred muscle. Specifically, transfer of flexor carpi ulnaris muscle success of tendon transfer surgeries (Khanna et al., 2009; (FCU) to the distal tendons of extensor carpi radialis brevis and Strickland, 2000). Such connective tissues are held responsible longus muscles (ECR) was studied in the rat. We hypothesized for unexpected outcomes following transfer of rectus femoris (RF) that due to the presence of scar tissue, not all force generated by myofibers of transferred FCU is transmitted onto its new insertion. Accordingly, dissection of its distal tendon and muscle n Corresponding author. belly will change the transmission of FCU force onto its distal E-mail address: [email protected] (H. Maas). tendon.

0021-9290 & 2011 Elsevier Ltd. Open access under the Elsevier OA license. doi:10.1016/j.jbiomech.2011.10.026 290 H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296

2. Methods threads in series with steel rods, the distal tendons were connected to a force transducer (BLH Electronica, Canton, MA, USA; maximal output erroro0.1%, compliance of 0.0162 mm/N). The orientation of the forelimb in each group was 2.1. Animals adjusted to enable force measurement in the muscle’s line of pull (Fig. 1). In each experiment several sets of FCU length–force data were collected: Data were obtained from 12 male Wistar rats that were divided in a tendon (1) with minimally disrupted connective tissues (i.e., the condition after surgical transfer group (n¼7) and a control group (n¼5). Surgical and experimental procedures), (2) after full dissection of FCU distal tendon exclusively, and (3) after procedures were in strict agreement with the guidelines and regulations concerning additional dissection of the distal half of FCU muscle belly, leaving the muscle’s animal welfare and experimentation set forth by the Dutch law and were approved primary blood supply and innervation intact. Dissection was performed in such a by the Committee on Ethics of Animal Experimentation at the VU University. way that FCU muscle belly and tendon were not damaged (for details see Supplementary materials).

2.2. Surgical procedures for FCU-to-ECR tendon transfer

2.5. Length–force characteristics of FCU muscle Tendon transfer surgery was performed under aseptic conditions, while the rats (body mass at the time of surgery 181715 g) were deeply anesthetized Before acquiring data, FCU muscle and structures linking muscle belly and (respiration of 1–3% isoflurane). At the same time, the rats were treated with a tendon to its surroundings were preconditioned by several isometric contractions single dose (0.03 ml) of buprenorphin (Temgesics, Schering-Plough, Maarssen, near optimum length to minimize effects of previous activity at high length The Netherlands; 0.3 mg/ml solution) for pain relief. Body temperature was (Huijing and Baan, 2001). monitored and the anesthetic state was checked routinely by evaluating with- FCU was excited by supramaximal stimulation of the ulnar and median nerves drawal reflexes. The distal tendon and distal half of the muscle belly of FCU was via a cuff electrode connected to a constant current source (0.3 mA, pulse width dissected free and transferred to the distal tendons of ECR. Details of surgical and 100 ms). Isometric forces of FCU were measured at various muscle–tendon experimental procedures are presented in Supplementary materials. complex lengths ð‘mþtÞ. To minimize effects of stress relaxation in intra- and epimuscular connective tissues, FCU was excited up to the length that corre- 2.3. Surgical procedures in preparation for assessing FCU mechanical characteristics sponded to a passive force of maximally 0.2 N. In the tendon transfer group, a marker was placed on the most distal end of FCU muscle belly to assess changes of muscle belly length ðD‘mÞ (Fig. 1B). In the Five weeks after tendon transfer, FCU length–force characteristics were assessed. control group, prior to dissection the muscle belly of FCU is covered entirely by the Body mass of the tendon transfer group at this time of the experiment was 356717 g. antebrachial fascia and, hence, ‘m could not be assessed. Marker position was For the control group, body mass at the time of assessing FCU characteristics was recorded before and during isometric contractions at each ‘mþt using a digital similar (i.e., 35975 g; not significantly different from transfer group). camera (Sony HC53E Digital Camcorder, 25 frames/s, 720 76 pixels, resolution The right forelimb of deeply anesthetized (intraperitoneally injected urethane) 1 pixel0.1 mm). rats was shaved and the skin was resected from the shoulder to the wrist. To assess FCU length–force characteristics, the distal tendon of FCU was identified, released from the skeleton, and attached to Kevlar thread. Within the brachial compartment, the ulnar and median nerves were identified and placed in a bipolar cuff electrode. 2.6. Treatment of data and statistics

Mathematical functions were fitted to the experimental data for further 2.4. Experimental conditions treatment and averaging (see Supplementary materials). To assess the effects of tendon and muscle belly dissection, ‘mþt was expressed as the deviation from The rat was placed on a heated platform and the right forelimb was secured the length corresponding to zero total force exerted at the distal tendon in the rigidly to the experimental setup (for details see legend Fig. 1). Using the Kevlar minimally disrupted condition.

(ventral view) (dorsal view)

Transferred FCU

FCU Marker on FCU

Clamp on humerus Clamp on humerus

Δ m+t FCU Δ m+t FCU

F-FCU F-FCU

Fig. 1. Schematic view of right forelimb in experimental setup. The right forelimb was secured rigidly by clamping the humerus and by firmly tying the manus to analuminum plate with 1-0 silk suture. The forearm was secured in horizontal position, the wrist in neutral position (i.e., 1801 flexion), and the elbow joint at approximately 901. The distal tendon of m. flexor carpi ulnaris (FCU) was connected to a force transducer, which was mounted on a single axis micropositioner. In the experiment, the muscle–tendon complex length of FCU was varied ðD‘mþtFCUÞ. The connective tissues enveloping the muscle bellies were left intact. In the tendon transfer group (B), a marker (K) was placed at the most distal end of the FCU muscle belly. Orientation of body and forelimb in each group was adjusted to enable force measurement in the direction of the muscle’s line of pull. In the control group (A), the rat was lying supine (i.e., body to the left of the schematic; see drawing whole rat, mouth visible) and the lower arm was secured with the palmar side of the manus and forearm faced upwards. Thus, a ventral view of the forearm showing the wrist flexors and the palmar side of the manus is shown. In the tendon transfer group (B), the rat was lying prone (i.e., body to the right of the schematic; see drawing whole rat, eyes visible) and the lower arm was secured with the dorsal side of the manus and forearm facing upwards. Thus, a dorsal view of the forearm showing the wrist extensors and the dorsal side of the manus is shown. H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296 291

Two-way ANOVAs for repeated measures (factors: ‘mþt and stage of dissec- indicate that the tendon of control FCU is linked mechanically to tion) were used to analyze effects of tendon and muscle belly dissection on surrounding tissues. (i) passive and total forces of transferred and control FCU, (ii) muscle belly length of transferred FCU, and (iii) the length at which maximal total force was found Length-total force characteristics were affected by tendon and ð‘maxÞ in control FCU. Comparisons between conditions were performed exclu- muscle belly dissection (Fig. 2B). Post-hoc analysis revealed (i) a sively for the lengths that were measured in all three stages of dissection (i.e., significant force decrease after tendon dissection (by a mean of 0oD‘mþto6:0mm in the control group and 0oD‘mþto1:2mm in the tendon 10% at 2:5oD‘mþto3:0mm), (ii) a significant additional transfer group). If significant interaction effects were found, Bonferroni post hoc decrease after muscle belly dissection (by a mean of 8% at tests were performed to locate significant differences at specific lengths. To assess effects of scar tissue formation following tendon transfer, slopes of length–passive 3:0oD‘mþto6:0mm), and (iii) significant differences between force curves for three given levels of force (i.e., 0.02 N, 0.04 N and 0.06 N) and for the minimally disrupted and muscle belly dissected condition at the different stages of dissection of transferred FCU were compared to those of 1:5oD‘mþto5:0mm. In addition, tendon dissection produced control FCU using t-tests. P valueso0.05 were considered significant. significant shifts of ‘max to higher lengths (from D‘mþt ¼ 5:570:3mm to D‘mþt ¼ 5:870:2mm). Muscle belly dissection produced no additional shifts of ‘max. 3. Results These results indicate that connective tissue linkages at the tendon and muscle belly affect length–force characteristics of 3.1. Control FCU control FCU. As such dissections are performed also during surgery to enable a smooth FCU-to-ECR tendon transfer, this ANOVA indicates significant main effects on total and passive shows that the mechanical properties of FCU are changed acutely forces for factors FCU length and stage of dissection, as well as during surgery. interaction between these factors. Thus, length–force characteristics were affected significantly by the progressive stages of dissection (Fig. 2A and B). Post hoc analysis showed that at the highest lengths 3.2. Transferred FCU tested ðD‘mþt ¼ 4:526:0mmÞ passive forces were decreased by freeing the distal FCU tendon (from 0.0870.01 N to 0.0570.01 N at During dissection of FCU tendon and muscle belly, scar tissue D‘mþt ¼ 6:0mm). In addition, the slope of the length–force curve providing myofascial pathways between the boundaries of trans- at D‘mþt ¼ 6:0mm decreased from 0.06 N/mm to 0.03 N/mm. ferred FCU and its surroundings were observed. Scar tissue was Partial muscle belly dissection did not produce additional changes most clearly visible around the distal tendon (Fig. 3), but present of passive length–force characteristics (Fig. 2A). These results also at the muscle belly.

0.12 Minimal disruption Minimal disruption * 4 * 0.10 Tendon dissected Tendon dissected FCU (N) * Muscle belly dissected FCU (N) Muscle belly dissected 0.08 3

0.06 2 0.04 1 0.02

0.00 Total force - control Passive force - control 0 -101234567 -1 0 1 2 3 4 5 6 7 Δ Δ m+t FCU (mm) m+t FCU (mm)

0.12 Minimal disruption 2.8 Minimal disruption Tendon dissected * * FCU (N) 0.10 2.4 Tendon dissected Muscle belly dissected FCU (N) * 2.0 Muscle belly dissected 0.08 1.6 0.06 1.2 0.04 0.8 0.02 0.4 0.00 0.0 -101234Total force - transferred -101234 Passive force - transferred Δ Δ m+t FCU (mm) m+t FCU (mm)

Fig. 2. Effects of tendon and muscle belly dissection on length–force characteristics. Mean passive forces (A, C) and total forces (B, D) of control FCU (upper panels) and transferred FCU (lower panels) are plotted as a function of FCU muscle–tendon complex length ðD‘mþtÞ. Muscle–tendon complex length is expressed as the deviation from the length corresponding to zero total force exerted at the tendon in the initial condition (i.e., minimal disruption). This length was 31.270.8 mm in the control n group (n¼5). For the tendon transfer group, this length was 30.771.1 mm (n¼7). Significant difference (po0.05) between conditions of length–force characteristics as indicated by repeated measures ANOVA (for post-hoc analysis see main text). Comparisons between conditions were performed exclusively for the lengths that were measured in all three stages of dissection. Mean data were calculated exclusively for those lengths for which data from all rats are available. Therefore, the highest mean passive force shown is lower than the pre-set maximum of 0.20 N. 292 H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296

ANOVA indicates significant main effects on total and passive forces for FCU length and stage of dissection. Interaction between these factors was significant for passive forces exclusively. FCU was excited up to the length that corresponded to a passive force of maximally 0.2 N (see Section 2). This criterion resulted in a small range of FCU lengths tested in the initial condition, as well as in different length ranges for each of the three experimental conditions (Fig. 2C and D). Tendon dissection changed passive length–force characteristics significantly (Fig. 2C). Post hoc analysis showed that at the higher lengths tested ð1:0oD‘mþto1:2mmÞ, passive force was decreased (from 0.0470.04 N to 0.0270.01 N at D‘mþt ¼ 1:0mm). In addi- tion, the slope of the length–force curve at D‘mþt ¼ 1:0mm was reducedtenfold(from0.10N/mmto0.01N/mm).Length–forcedata further indicate that the length at which passive force approaches zero was decreased after tendon dissection. Partial muscle belly dissection did not produce significant additional changes of passive length–force characteristics. For the length range studied in the minimally disrupted condition (i.e., 0 mþt 1:2mm), post hoc analysis revealed Fig. 3. Dorsal view of an exemplar forearm after removing the skin exclusively, oD‘ o showing scar tissue near the distal tendon of transferred FCU. that dissection of the tendon increased total force significantly

Minimal disruption 20 20 Tendon dissected Muscle belly dissected

18 18

16 16 (mm) (mm) ma mp

14 Minimal disruption 14 Tendon dissected Muscle belly dissected 12 12 -1 0 1 2 34-1 0 1 2 34

Δ m+t FCU (mm) Δ m+t FCU (mm)

1. FCU 1. FCU

Proximal Distal Proximal Distal

2. FCU 2. FCU

3. FCU 3. FCU

Fig. 4. Effects of tendon and muscle belly dissection on muscle belly length of transferred FCU. (Top) Muscle belly length of transferred FCU in passive state ð‘mp; AÞ and during isometric contraction ð‘ma; BÞ as a function of FCU muscle–tendon complex length ðD‘mþtÞ in the three conditions of dissection. Muscle–tendon complex length is expressed as the deviation from its length corresponding to zero force exerted at the tendon in the initial condition (i.e., minimal disruption). Values are shown as means and SD. Muscle belly length data were available for six of seven rats tested (n¼6). (Bottom) Schematic of FCU muscle–tendon complex kept at D‘mþt ¼ 0mm prior to (C) and during isometric contraction (D) illustrating additional muscle belly shortening during isometric contraction after tendon and muscle belly dissection. Three conditions are shown: (1) the minimally disrupted, (2) tendon dissected, and (3) muscle belly dissected conditions. H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296 293

(from 0.4470.23 N in the minimally disrupted condition to the slope of the curve increased at low lengths (Fig. 4A and B). 0.6970.34 N after tendon dissection at D‘mþt ¼ 1:0mm). For the minimally disrupted condition, dissected tendon, and Another significant increase in total force was found following dissected muscle belly conditions, lengthening of FCU (from muscle belly dissection (to a value of 0.8570.34 N at ‘mþt ¼ 0 to 1.0 mm) increased ‘mp by 0.12, 0.38, and D‘mþt ¼ 1:0mm) (Fig. 2D). The length corresponding to zero 0.68 mm, respectively, and ‘ma by 0.12, 0.42, and 0.61 mm. total force exerted at the tendon was decreased after dissection. These results indicate that both tendon and muscle belly of ANOVA indicates significant main effects for factor stage of transferred FCU were linked mechanically to surrounding struc- dissection exclusively on FCU muscle belly length prior to and tures. Epitendinous and epimuscular connective tissues seem to during isometric contraction (‘mp and ‘ma, respectively). For the keep passive FCU muscle at above slack length (Fig. 4A) and limit length range studied in the minimally disrupted condition (i.e., FCU muscle belly shortening during contraction (Fig. 4B). The 0oD‘mþto1:2mm), ‘ma was significantly smaller following finding that high forces could be exerted on the dissected tendon tendon dissection, and decreased further after subsequent muscle without breakage indicates that our tendon transfer procedures belly dissection (Fig. 4). Therefore, identical D‘mþt at each stage resulted in a strong FCU-to-ECR tendon connection. of dissection involved different muscle belly lengths. In addition, 3.3. Comparison between control and transferred FCU 0.20 Transferred FCU In the minimally disrupted condition, we found significant Control FCU 0.16 differences between transferred and control FCU in the slope of the length–passive force curve (Fig. 5). At all force levels tested, the slope was significantly higher (3–4 times) in transferred 0.12 FCU than in control FCU. After tendon dissection, however, this * slope was equally steep in the control and transferred group. 0.08 The higher resistance to lengthening of transferred FCU indi-

at 0.02 N (N/mm) cates a higher stiffness of the tissues linking FCU to its surround- 0.04 ings after recovery from tendon transfer. Similarity in slope between groups following tendon dissection suggests a substan-

Slope length-passive force curve tial effect of scar tissue at the tendon. 0.00 Minimal Post tendon Post muscle disruption dissection belly dissection 4. Discussion

0.20 Transferred FCU A novel aspect of the present study is that we measured mechanical effects of dissection of connective tissue and scar Control FCU 0.16 tissue formed during recovery from tendon transfer. As hypothe- * sized, if stiff enough, such epitendinous and epimuscular linkages 0.12 redirected part of the force from transferred FCU to structures other than its distal tendon. The finding that length–force char- acteristics of control FCU were affected also by connective tissues 0.08 at the tendon and muscle belly indicates that some connectivity

at 0.04 N (N/mm) should be classified as physiological (i.e., present in the unaf- 0.04 fected musculoskeletal system). Other studies have provided indirect evidence of comparable Slope length-passive force curve 0.00 mechanical effects of scar tissue formation following tendon Minimal Post tendon Post muscle transfer. The present results are in agreement with those from disruption dissection belly dissection Delp and colleagues who found that scar tissue, linked to transferred muscle, can redirect some muscle force to adjacent structures (Asakawa et al., 2002; Gold et al., 2004; Riewald and 0.20 * Transferred FCU Delp, 1997). Control FCU 0.16 4.1. Post-dissection length–force characteristics

0.12 We found effects of tendon dissection on passive and total forces of control FCU. Mechanical effects of epitendinous tissues 0.08 were first described for rat medial gastrocnemius and plantaris muscles (Rijkelijkhuizen et al., 2005). They showed that connec- at 0.06 N (N/mm) 0.04 tive tissue around the tendon is capable of transmitting muscle forces directly to bone bypassing the tendon of insertion. Present

Slope length-passive force curve effects of muscle belly dissection in the control group confirm 0.00 previous findings in rat FCU (Smeulders et al., 2002), indicating Minimal Post tendon Post muscle disruption dissection belly dissection effects of epimuscular myofascial force transmission. As any muscle belly dissection involves fasciotomy and dissection of Fig. 5. Comparison between transferred and control FCU of slopes of length– intermuscular connective tissues, the epimuscular pathway is passive force curve for given levels of force. A comparison between groups was likely constituted by both structures (Maas et al., 2005). Similar made at three force levels (i.e., 0.02 N, 0.04 N and 0.06 N) that were found in n effects of muscle–tendon dissection have been reported for transferred and control FCU and all three dissection conditions. Significant difference between transferred and control FCU. Values are shown as means and several upper extremity muscles measured in cadaveric speci- SD (n¼7). mens (Friden et al., 2001, 2006) and have been measured also 294 H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296 during tendon transfer surgery in human patients (Smeulders and Increased total forces after dissection in transferred FCU are Kreulen, 2007). We conclude that surgical procedures, standard contrasted by force decreases in controls. At first sight, the results for tendon transfers, acutely affect mechanical properties of the of the control group seem compatible with increased tendon muscle to be transferred. compliance causing lower forces at equal muscle–tendon com- Effects of muscle belly and tendon dissection were enhanced plex lengths due to additional myofiber shortening (Zajac, 1989). in transferred FCU compared to controls (see Fig. 5). This suggests Without involvement of epitendinous and/or epimuscular lin- that scar tissue formation between FCU and tissues of its new kages, any conceivable direct effect of dissection on series environment increased the stiffness of epitendinous and epimus- elasticity must be brought about by damage to FCU tendon. cular linkages, resulting in myofascial connectivity being different However, because of the way the tendon was dissected (i.e., at from the physiological condition. As scar tissue was disrupted in the expense of surrounding tissues; see supplementary materials), the present study during the process of dissection, the volume of systematic changes in tendon compliance exclusively as a con- scar tissue and its histological and mechanical properties could sequence of dissection are deemed highly unlikely. not be assessed. Alternatively, dissection affected epitendinous and epimuscu- In both groups, tendon dissection decreased passive FCU force, lar linkages, which exert loads on the muscle. Effects of dissecting as well as the slope of the passive length–force curve. This can be tendon and muscle belly on force transmission during isometric explained by removal of proximally directed epitendinous loads contraction of transferred and control FCU are illustrated sche- on FCU tendon (i.e., additional forces exerted by scar tissues on matically (Fig. 6). the tendon in proximal direction), present at higher lengths of As the force decreases found in the control group were FCU in the minimally disrupted condition (Maas et al., 2001, significant at higher lengths ð2:5oD‘mþto6:0mmÞ, we expect 2004). a net proximally directed load on FCU tendon and muscle belly

Fepimuscular (net distal load)

Proximal To the wrist 2

FCU 1 Proximal To the wrist

Fepitendinous (net distal load)

1. Disrupting epitendinous linkages: Ft ↑ to 0.17 N 2. Disrupting epimuscular linkages: Ft ↑ to 0.35 N

Fepimuscular (net distal load)

Proximal To the wrist 2

FCU 1 Proximal To the wrist

Fepitendinous (net distal load)

1. Disrupting epitendinous linkages: Ft ↓ to 2.3 N 2. Disrupting epimuscular linkages: Ft ↓ to 2.1 N

Fig. 6. Schematic illustrating effects of tendon and muscle belly dissection on total force exerted at the distal tendon of transferred (A) and control (B) FCU. In agreement with the conditions during the experiments, three components of FCU are distinguished that are present initially: (i) muscle belly with connective tissue linkages, (ii) tendon with connective tissue linkages, and (iii) free tendon. Epimuscular and epitendinous loads (i.e., additional forces exerted by connective tissues on the muscle belly and tendon) are represented as springs, the higher the length of a spring, the higher the load exerted on FCU via it. The minimally disrupted condition is shown. Subsequent dissections of (1) tendon and (2) muscle belly are indicated by gray crosses labeled accordingly. (A) Transferred FCU. Muscle is kept at D‘mþt ¼ 0mm (i.e., length corresponding to zero total force (Ft) exerted at the tendon in the minimally disrupted condition). In the condition with minimally disrupted connective tissues, both epimuscular and epitendinous linkages exert a net distally directed load on FCU. These loads are so high that no force is exerted at the insertion, but all force generated by FCU myofibers is exerted via non-tendinous connective tissues. Tendon dissection eliminates loads exerted on FCU via epitendinous linkages. Part of FCU force, transmitted previously to the skeleton via distally directed epitendinous linkages, is now exerted at FCU distal tendon (Ft m). The remaining epimuscular links still exert net a distally directed load on FCU. Muscle belly dissection eliminates forces exerted on FCU via epimuscular linkages. More force is exerted at the distal tendon (Ft m). Now all force exerted by FCU myofibers is exerted at the tendon. (B) Control FCU. Muscle is kept at D‘mþt ¼ 3mm (i.e., length at which significant decreases in total force were found). In the condition with minimally disrupted connective tissues, both epimuscular and epitendinous linkages exert a net proximally directed load on FCU. Such loads can be the result of forces generated by myofibers of surrounding muscles. Tendon dissection eliminates epitendinous loads and, hence, the transmission of forces from surrounding muscles to the distal tendon of FCU (Ft k). The remaining epimuscular links still exert net a proximally directed load on FCU. Muscle belly dissection eliminates the transmission of forces from surrounding muscles to the distal tendon of FCU via epimuscular linkages (Ft k). H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296 295

(see Fig. 6B). This is based on several previous studies indicating (Buchmann et al., 2011; Carpenter et al., 1998). Nevertheless, it that for a muscle, which is positioned distally relative to its was shown in the rat that 3–6 weeks after a complete transection surroundings, additional force is transmitted from neighboring of the supraspinatus tendon the conditions of muscle, tendon and muscles to its distal tendon (Maas et al., 2001). Disrupting scar tissues were comparable to those after adaptation to lesions epitendinous and/or epimuscular connections in such a condition of human rotator cuff tendons (Buchmann et al., 2011). (i.e., removing the proximally directed load on FCU) is equivalent Due to differences between rats and humans, the results of the to removing a force that was transmitted from sources other than present study may not be transferred directly to human medical FCU onto its distal tendon and, thus, will cause a drop of force. conditions. Nevertheless, our findings do provide new insights This will be counteracted partly by slightly higher lengths of FCU into mechanical effects of connective and scar tissues, of which myofibers and, hence, higher forces generated by them, due to a the general concepts are relevant also for understanding muscle decreased length of FCU distal tendon. function following tendon transfer in human patients. The increase in total force of transferred FCU was found at ð þ Þ relatively low lengths 0oD‘m to1:2mm . At the length 4.3. Musculoskeletal modeling shown in Fig. 6A ðD‘mþt ¼ 0mmÞ, no force is exerted at the tendon in the minimally disrupted condition. However, this does Present results indicate that due to the presence of scar tissue, not necessarily mean that FCU myofibers did not exert force. They not all force generated by myofibers of transferred FCU may be may be prevented from shortening to active slack length (zero transmitted via its distal tendon and exerted at the new insertion. force) by distally directed epimuscular and/or epitendinous loads. Consequently, the actual mechanical effects of transferred FCU on This is in agreement with the finding that passive FCU muscle the wrist joint for the three anatomical axes (i.e., flexion– belly shortened following dissection (Fig. 4), as well as with our extension, eversion–inversion, and abduction–adduction) will previous observation that after severing the new insertion of differ from its effect predicted if based solely on the new moment transferred FCU, intramuscular excitation still yielded wrist arm at the joint. Therefore, biomechanical models of the muscu- extension movements (Maas et al., 2010). loskeletal system that do not take into account effects of newly If the net load is directed distally, part of FCU force will be formed connective tissues at muscle–tendon boundaries are not transmitted via epitendinous and epimuscular linkages in distal suitable to fully assess effects of tendon transfer surgeries. direction and this part cannot be exerted at the FCU insertion. Note that this net load acts in series with (parts of) FCU myofibers. Dissection should cause (1) an increase in the propor- Conflict of interest statement tion of FCU force that is transmitted to the FCU insertion. In addition, dissection should cause (2) an increase in the compli- The authors declare no competing financial interest and ance of the myofiber in series elasticity. The latter will result in personal relationships with other people or organizations that more shortening of those sarcomeres that are no longer exposed could inappropriately influence the work presented in the sub- to this reaction force (MacIntosh and MacNaughton, 2005; Zajac, mitted paper. 1989) and, hence a lower force generated by FCU myofibers at equal muscle–tendon complex lengths. It should be noted that higher total forces were found despite a significantly reduced Acknowledgments muscle belly length (Fig. 4). The above indicates that the force changes following tendon The authors thank Oscar van Kooten and Dr. Marco Ritt for and muscle belly dissection of control and transferred FCU were directions on the FCU-to-ECR tendon transfer surgery, and Guus most likely caused primarily by removal of epimuscular and Baan and Erwin van Gelderop for technical support. epitendinous loads. The research leading to these results was in part supported by the European Community’s Seventh Framework Programme 4.2. Relevance to human patients under Grant agreement MIRG-CT-2007–203846. HM is currently supported by the Division for Earth and Life Sciences with The rats were studied five weeks after the tendon transfer financial aid from the Netherlands Organization for Scientific surgery. This may appear a short time. However, if the much Research (Grant 864-10-011). shorter life span of rats (3 years) is taken into account, this compares to 2.5 years in humans. For comparison, rectus femoris muscle function has been evaluated 0.7–9.0 years after Appendix. Supplementary materials tendon transfer surgery (Asakawa et al., 2002). In addition, four weeks of immobilization of a limb in rats has proven to be long Supplementary materials associated with this article can be enough for causing major changes in muscle length–force char- found in the online version at doi:10.1016/j.jbiomech.2011.10. acteristics and muscle architecture (Heslinga and Huijing, 1993; 026. Heslinga et al., 1995). Even though there are differences in anatomy and physiology between humans and rats (e.g., Soslowsky et al., 1996; Warden, 2007), we did find scar tissue formation in response to tendon References transfer surgery. Scar tissue is found also following tendon transfers in human patients (Khanna et al., 2009; Strickland, Asakawa, D.S., Blemker, S.S., Gold, G.E., Delp, S.L., 2002. In vivo motion of the rectus 2000). This suggests that our model mimics an important aspect femoris muscle after tendon transfer surgery. Journal of Biomechanics 35, of the human condition. 1029–1037. Buchmann, S., et al., 2011. Rotator cuff changes in a full thickness tear rat model: However, connective responses to tissue damage in rats do not verification of the optimal time interval until reconstruction for comparison to seem to be equivalent to that in humans. Rats have been noted to the healing process of chronic lesions in humans. Archives of Orthopaedic and have a higher healing potential (Glasser et al., 2007). In contrast Trauma Surgery 131, 429–435. Carpenter, J.E., Thomopoulos, S., Flanagan, C.L., DeBano, C.M., Soslowsky, L.J., 1998. to humans, complete self-healing of supraspinatus tendon Rotator cuff defect healing: a biomechanical and histologic analysis in an 9 weeks after distal tenotomy has been reported in rats animal model. Journal of Shoulder and Elbow Surgery 7, 599–605. 296 H. Maas, P.A. Huijing / Journal of Biomechanics 45 (2012) 289–296

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