- Email: [email protected]
Soft tissue responses to limb lengthening
~
J Orthop Sci (1997) 2:191-197
]ournal of
thopaedic Science The Japanese Orthopaedic Association
Instructional lecture
Soft tissue responses to limb lengthening K o z o NAKAMURA, TAKASHI MATSUSHITA,HIROSHI OKAZAKI, and TAKAHIDE KUROKAWA Department of Orthopaedic Surgery, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Abstract: The responses of soft tissues to limb lengthening or stretching are: reduction in physiological slack, tissue migration from the adjacent region, stress relaxation (as occurs in viscoelastic materials), tissue formation, and tissue injuries. Theoretically, an optimal condition for limb lengthening is that under which new tissues are formed without injury. However, phenomena, such as angiogenesis in the dermis, due to tissue hypoxia, suggest that injury triggers tissue formation; the conditions under which tissue formation and tissue injury occur are not always opposed. Components of the soft tissues of the leg have their own distinctive functions, biological activity, and biomechanical properties, so that the relationship between tissue injury and functional impairment differs among these tissues. Very little information on limb lengthening in humans is available, in terms of the mechanism of soft tissue lengthening. Further work is needed to determine the optimal lengthening conditions for each soft tissue. Key words: limb lengthening, muscle, peripheral nerve, skin, stretch
Introduction
Limb lengthening is used to correct limb length discrepancy and to increase short stature. As limbs consist of bone and soft tissues, limb lengthening involves the lengthening of both these components. Bone response can be evaluated by various methods, for example, by X-ray and dual energy X-ray absorptio-metry. Many factors have been studied to establish clinical guidelines for bone lengthening and problems with bone formation can now be controlled well. 1~,17,32However, clinical methods for evaluating soft tissue responses to distrac-
Offprint requests to: K. Nakamura Received for publication on Nov. 21, 1996; accepted on Dec. 16, 1996
tion have not yet been established, and problems relating to these tissues often cause clinical complications such as joint contracture, deformity of the lengthened bone, and neurological complications. 32 In limb lengthening, bone is initially osteotomized and a type of repair process is responsible for bone formation during lengthening. Since soft tissues are not cut before lengthening begins, a biological process different from that in bone is thought to be involved. Skin, nerve, muscle, and other components of the soft tissues are viscoelastic, 14,2~ that is, they exhibit timedependent behavior. When the rate of loading increases, the modulus increases. When the tissue is held at a constant deformation, load decreases (stress relaxation). There is no doubt that this viscoelastic behavior is, at least in part, responsible for the lengthening of soft tissue. During lengthening, tensile force repeats a cycle of instantaneous increase after distraction, followed by reduction, mimicking stress relaxation. When lengthening was performed twice a day, 0.5 mm at 8 a.m. and 8 p.m., the reduction rate during the 12 night hours was significantly higher than that during the daytime (Fig. 1). 26 Since non-living viscoelastic material shows no difference within a period of i day, this difference is thought to reflect some biological response. One possibility is the growth of tissues.
Peripheral nerves
Responses to limb lengthening The magnitude of nerve elongation is not consistent with that of bone lengthening. In the rabbit, elongation of the femoral nerve (from the sciatic notch to the bifurcation of the peroneal nerve) for 10% and 20% femoral lengthening was 4.5 % and 11.7 %, respectively? Elongation of nerves occurs not only at the site of the
192
K. Nakamura et al.: Soft tissue responses to limb lengthening
k9
10
0 0
I 2
I 4
I 6
I 8
osteotomy but also throughout the nerves. In this animal, femoral nerve elongation occurred equally in the proximal, middle, and distal segments. 9 Nerves are highly viscoelastic tissues and show m a r k e d stress relaxation. The mean 1-h stress relaxation of rabbit tibial nerves stretched for 6%, 9%, and 12% of their original resting length was 48%, 34%, and 34%, respectively. 42 U n d e r constant elongation of rat sciatic nerve produced by 30-g tension, the tension was reduced by 30% in the first 10min and by a small amount in the ensuing 20 min. 2~ Increase of internodal length is reported to be one response to limb lengthening; internodal length increased by 11% in 18.5% femoral nerve elongation with femoral lengthening conducted at the rate of 0.5-ram twice a day. 47 Ilizarov 17 reported nerve fibers at various stages of formation and differentiation as evidence of new nerve formation during tibial lengthening in dogs of 0.25 mm every 6 h or less often. It has been pointed out that the frequency of peripheral nerve complications during limb lengthening is underestimated and that peripheral nerve dysfunction can be detected more frequently by electrophysiological tests: compound muscle action potential 6 and somatosensory-evoked potential 22 having been used in rabbit tibial lengthening, and electromyography and motor conduction velocity in human tibial lengthening? ~ The incidence of neurological dysfunction depended on the rate and frequency of distraction. Murashima et al. 27 measured the motor nerve conduction velocity (MCV) of the deep peroneal nerve during tibial lengthening in humans. A decrease in velocity (to less than 40m/s) was seen in 14 of 18 nerves at 3 0 % - 9 8 % lengthening done manually at the rate of more than 0.5 mm/ day in two steps per day. However, even when the limb was lengthened in two steps per day, there was no de-
i I Odays
Fig. 1. Time course of tensile force during limb lengthening.26 Femoral lengthening of 0.5 mm was performed twice a day, at 8 a.m., and 8 p.m., in a 12-year-old girl. Force was increased progressively, repeating a cycle of increase and reduction. Note that the reduction rate of the force during the 12 night hours was higher than that during the daytime. Circles, Morning; dots, evening
crease of MCV in nerves in which the mean distraction rate was less than 0.5mm per day. Further, there was no decrease of MCV in nerves in which distraction, at a frequency of 1440 steps per day, was produced by an autodistractor (Hifixator system; Nagano Keiki, Nagano, Japan), even if the limb was lengthened at the rate of 0.5-1.0mm/day. Ilizarov 17 reported that at a distraction rate of l mm once every 24h, nerve fibers exhibited uneven axon diameter and formation of irregular accumulations of cytoplasm, but at a distraction rate of 0.017mm every 24rain using his autodistractor, the nerve fibers had a normal structure. Major nerve injury complications also depend on the amount of lengthening; they rarely occur with lengthening of less than 10cm, but with lengthening of more than 10cm, 12% of patients developed transient sensory or motor loss. 32This close relationship between the amount of distraction and nerve dysfunction was confirmed in the rabbit tibial nerve during tibial lengthening. 22
Responses to acute stretching During acute stretching, within the physiological range of limb motion, the nerve may initially, stretch by up to 15% strain under minimal tension, much of the elongation being due to removal of the slack in the fascicles and in the nerve trunk. 36 Normal nerves have a wavy, sinusoidal shape in longitudinal section. On stretching, this sinusoidal shape is lost, 36the cross-sectional area of the funiculi is markedly reduced, and fibers become straight and closely packed. 12 The strain-stress curve (strain rate, 0.5%/s) of the rabbit tibial nerve showed low stiffness up to about 15%. The linear stiffness was reached at about 20% tensile strain and remained unchanged until failure. 36
K. Nakamura et al.: Soft tissue responses to limb lengthening For the rabbit tibial nerve, the ultimate strain was reported to be 39% at a strain rate of 0.5%/s, and 3673% at a strain rate of 0.5 mm/min 12, and the ultimate load was 7-10 N. 36 For human ulnar nerve, the ultimate load was 65-155 N? 6 There were multiple ruptures of perineurial sheaths when the nerve failed mechanically, indication that the perineurium may be a major load-carrying tissue component. 36 The importance of sufficient blood supply for nerve function is well known. 25As to changes in intraneural microcirculation due to acute stretching, a slowing down in venular flow was seen at 5 % - 1 0 % elongation of the rabbit tibial nerve, and complete cessation of arteriolar and capillary flow occurred at 11%-18% elongation. 25 Other authors have reported that 16% stretching caused complete arrest of blood flow in the rabbit sciatic nerve 3~ and the rabbit tibial nerve. 4~ The influence of elongation on the intraneural microvascular flow may be due to the reduced cross-sectional area, causing an increase in intrafascicular pressure and consequent interference with intrafascicular nutritive blood flow. 25 Nerve dysfunction was observed even when no morphological change was detectedY ,43 The functional changes in a stretched nerve are highly dependent on the magnitude and character of the deforming force, as well as on the length of time during which the force operates. As little as 6% strain caused a 70 % decrease in action potential amplitude if it lasted for 1 h in the rabbit tibial nerve; the onset of deficits may be delayed. 43 Since this injury occurred at a strain lower than that which produced intraneural arterial occlusion, 25 it appears that mechanical deformation, not ischemia, may be responsible for the early dysfunction.
193
Skin
Responses to limb lengthening Skin is the only tissue whose changes can be observed directly during lengthening. In human patients, Hiraki et al. 13drew a grid pattern, with lines perpendicular and lines parallel to the axis of the leg, on the skin surface from 15cm above the knee to 10cm below the ankle. They measured changes in length and area during tibial lengthening, using the Hifixator (Nagaro Keiki) a unilateral type lengthener. At 60% bone lengthening, the increases in length of the skin on the anterior surface were: 68% midway between the two clamps, 60% at the distal clamp, 35% at the proximal claknp, 13% at the foot, and 10% above the knee (Fig. 2). Skin migration into the expanded field was noted: 8.Smm in the proximal direction of the ankle skin and 8.0 mm in the distal direction of the knee skin at 60 % bone lengthening. 13 Ilizarov 16 described histological findings of hyperplasia of basal cells, hair follicles, and sweat and sebaceous glands after tibial lengthening in dogs as evidence of histogenesis due to lengthening.
Responses to tissue expansion Normal skin has slack in the resting position. The epidermis has a two-stage response to progressive strain. Initially, its undulating form is pulled out flat and further stretching elongates the cells. In response to increasing strain, the dermal multidirectional system of wavy fibers is reorientated, becoming aligned and compacted? The mechanism of increase in skin surface area after expansion can be explained, at least in part, in terms of
% 8060tD
b (=
O-20
Region
i
30
A
60
0
30
60
0
30
60
0
30
% bone lengthening B C D
60
0
30
E
60
Fig. 2. Changes in skin length on the anterior surface of the leg in each region during tibial lengthening.13 A grid pattern drawn on the skin surface was analyzed from 15 cm above the knee to 10 cm below the ankle. At 60% bone lengthening, the increases were: 68% midway between the clamps (C), 60% at the distal clamp (D), 35% at the proximal clamp (B), 13% at the foot (E), and 10% above the knee (A). (n - 10)
194
K. Nakamura et al.: Soft tissue responses to limb lengthening
stress relaxation, because the skin is viscoelastic. The oxygen saturation of the skin dropped steeply for a period of up to 20s and then rapidly returned to its initial normal value within as stress relaxation took place minutes. As the expansion was repeated, however, the oxygen saturation recovery time became slower. TM Capillaries are created in the papillary dermis after tissue expansion, and tissue hypoxia due to distraction may contribute to the neovascularization.19 The inflation of a tissue expander resulted in a threefold elevation of epidermal mitotic activity within 24 h, 3 and a significant thickening of the epidermis of pig skin. 2,18,19,34On the other hand, the dermis and subcutaneous tissue were significantly thinner after expansion in humans 34and pigs. 18 With a chronic stretching force, fibroblasts in the dermis increased in number, and the rough endoplasmic reticulum of these cells contained large cisternae, indicating increased metabolic activity.33 Although dermis thinning persisted for 36 weeks after expansion in pig skin, the total collagen content, calculated within an expanded square grid, increased; the increase in surface area was not merely due to stretching of the skin but to new tissue having been created in the dermal layer because of the chronic stretching force. TM However, since the dermal thinning persisted, it seems that compensation is slower in this layer, in contrast to the rapid compensation seen in the epidermal layer. Although the gross dermal architecture remained unchanged, the collagen bundles in the expanded skin showed an increased diameter, with loosely packed collagen fibrils. In terms of biomechanical properties, the
expanded skin showed 60% less stiffness than before expansion, and deformation between maximal stiffness and failure increased by 20%. These findings indicate damage to the dermis. Therefore, the new tissue formation that takes place in the dermis because of expansion may be initiated by wound healing and repair. The normalization of the dermis 2 years after expansion reported by Pasyk et al. 34 seems to confirm this sequence of events. 4~
Muscle
Responses to limb lengthening Gain in length of the musculotendinous unit can be determined by evaluating the range of motion of the adjacent joint. The dorsiflexion angle of the ankle joint was maintained even at 60% tibial lengthening in patients in whom orthosis was used to maintain the dorsiflexion of the ankle for a period of more than 16h/ day; there was an actual gain in length of triceps surae due to tibial lengthening (Fig. 3). 29During tibial lengthening in rabbits (0.5mm, twice daily for 20 days) the weight of the tibialis anterior muscle increased significantly more on the lengthened side than on the contralateral side. 28 During distraction, the muscles lengthen throughout the entire muscle, and not simply at the level of the osteotomy.46A gain in length has also been observed in the tendinous portion, as well as in the muscle belly; with 2-cm lengthening, the former was 0.67 cm and the latter 1.22 cm.28Since musculotendinous
degree I
I
30 !
I
r-"l
i
20
!
I
I
I-'-1
!*i
N'8
10" tO X
N-8 N-6 N-10
0 -10
N'6 N-5
0 "0 tt~
N-E
-20
N-6
N-8
-30 -40
N-8 N-12
-50
-
10
20
30 % length gain
40
50
%
Fig. 3. Relationship between dorsiflexion angle of the ankle and percent tibial lengthening. Comparison of effectiveness of three methods in preventing an equinus deformity during tibial lengthening.29Circles, Orthosis _->16h/day; squares, orthosis <16 h/day; triangles, physiotherapy. *P < 0.05; **P < 0.01
K. Nakamura et al.: Soft tissue responses to limb lengthening
195
units respond viscoelastically to tensile loads, 39 stress relaxation must be one of the factors responsible for the increase in length. The results of previous studies vary regard in to the findings on the influence of limb lengthening on muscular fiber type: reports show a shift from type 1 to type 2 (sheep), 5 no change in the proportion of muscle fiber types (rabbit, gastrocnemius), 23 decrease in mean size of type 1 and 2 muscle fibers (rabbit, gastrocnemius), 23 type 1 fiber hypertrophy (rabbit, tibialis anterior), and 28 type 2 fiber atrophy (Sheep, biceps femoris). 24 These results may indicate the influence of muscular inactivity during limb lengthening, as well as the influence of continuous stretching. Muscle damage, such as internalization of nuclei and endomysial fibrosis, has also been reported. The severity of histological changes was reported to be related linearly to the percentage of lengthening; substantial changes occurred in groups subjected to 20% and 30% bone lengthening (rabbit, gastrocnemius), 23 or to lengthening of more than 11% (sheep, tibial lengthening). 5 Muscle damage also depended on the rate of distraction, so that it was seen in all tibialis anterior muscles of the rabbit when they were lengthened at a rate of 1 mm or more. Limbs distracted at higher rates tended to show dysfunction even if the amount of lengthening was the same. 37
creased immediately after stretching, gradually normalized within 3 weeks 45or 7 days (Fig. 4) 38 in mice. The n u m b e r of sarcomeres increased within 2 weeks in the mouse soleus muscle. 44 These findings suggest that the stretched muscles are lengthened by the addition of new sarcomeres. A t the myotendinous junction (MTJ) of tibialis anterior muscles stretched for 4 days, there were higher levels of myosin heavy chain (MHC) m R N A , greatly increased amounts of vinculin (a major component of myofibril attachment at the MTJ), and a large cytoplasmic space containing nascent filament assemblies. 8 These results support the idea that stretched muscles lengthen by adding sarcomeres at the MTJ. Other findings showed nascent fibers and myotubes at the ends of the muscles after 6 days of stretching. It is thought that these myotubes fuse with and become extensions of existing fibers, similar to observations during development. 8 The time course of changes in muscle cross-sectional area was different from that in length, suggesting that the underlying molecular events responsible for longitudinal and cross-sectional growth may be independent. 15 Stretch-induced enlargement of the muscles is due to both an increase in fiber area (hypertrophy) and to an increase in fiber number (hyperplasia). 1 Satellite cells are postulated to be the sole source of myonuclei, and adult muscles can increase their mass or repair an injury by recruiting satellite cells, which then fuse with existing fibers (hypertrophy) or fuse together to form new myotubes (hyperplasia). 21,35After injury, in the rat, the satellite cells take several hours to become mobilized, and cell division is highest 2-4 days later. 35 In the stretched muscle fibers, myosin m R N A is concentrated around the nucleus and in the region corresponding to disruption of the normal myofibrillar architecture, providing local contractile protein synthesis for myofibril
Responses to immobilization in the stretched position The responses of striated muscles to stretching have been studied in models in which the muscles were maintained in the stretched position by casting of the limb. Increases in length and cross-sectional area and changes in fiber type have been reported. When the muscle was stretched and held in the stretched position, the sarcomere length, which was in-
E v~
3.0-
I-7u J
2.9-
'"
2.8"
~
(9)
(10) * A
E E
0
'~
2.7-
~(8)
~(10) t(10)
~(10) ~(7)
fi "4 1.2
1.0
Control
;
;
.
5
.
.
pl i
.
7
IMMOBILIZATION PERIOD (days)
14
"I" I-(.D ZI,M
Fig. 4. Time course of changes in sarcomere length (dots) and muscle length (triangles) of the extensor digitorum longus muscle during sustained stretching.38The limbs of mice were immobilized with the ankle and toes in full plantar flexion with a plaster cast. Sarcomere length was measured in situ by the light diffraction method. The sarcomere lengthened immediately after stretching and normalized gradually by 7 days. The muscle length showed no significant change during the period of immobilization. Values are means _+ SEM. *P < 0.05; **P < 0.01. Numbers in parentheses indicate numbers of animals
196
K. Nakamura et al.: Soft tissue responses to limb lengthening
assembly and repair. 35 The repeated injury of muscle fibers, followed by regeneration, may produce an overcompensation of protein synthesis, resulting in a net anabolic effect. 1 G r o w t h factors appear to be involved in the activation of satellite cell proliferation and availability for the repair process. Fibroblast growth factor (FGF) stimulates cell proliferation, whereas transforming growth factor-~ (TGF-~) has the opposite effect, supressing the proliferation of cultured satellite cells in vitro. Insulinlike growth factor (IGF) overrides the effects of TGF-~ and stimulates the proliferation of satellite cellsY Czerwinski e t al. 7 showed that, after 11 days of stretching the patagialis muscle of the chicken, the muscle weight increased by 60% and IGF-I m R N A levels increased threefold, suggesting a role for IGF-I in muscle hypertrophy. In striated muscle, a variety of mechanical stimuli have been shown to lead to a rapid and transient increase in the expression of a number of cellular oncogenes. A 15% stretching of the rabbit latissimus dorsi muscle caused peak expression of c-fos and c-jun m R N A 1 h after the imposition of stretching, determined by reverse transcription polynerase chain reaction. Immunostaining with anti-Fos and anti-Jun antibodies revealed the accumulation of these proteins in both myofiber and interstitial cell nuclei after passive stretching. These findings suggest that rapid induction of c-fos and c-jun is an early event in response to mechanical stretching and may trigger events leading to muscle fiber hypertrophy. 31 Slow-contracting and fast-contracting fibers are characterized by different protein isoforms, and the differentiation into fast or slow type fibers involves regulation of the expression of different subsets of genes. Stretching of the tibialis anterior muscle in an adult rabbit caused gene switching, which involved repression of a fast and activation of a slow myosin heavy chain gene, together with rapid hypertrophy within a period as short as 4 days. 11 This switching seems to be a reasonable adaptation, as the slow myosin heavy chain has lower specific ATPase activity and is therefore more economical for maintaining isometric force.
tissue formation, for example, angiogenesis in the dermis due to tissue hypoxia, dermis formation after tissue expansion, and muscle hypertrophy or hyperplasia that occurs via the recruitment of satellite cells. These p h e n o m e n a indicate: (i) that conditions for tissue formation and injury are not always opposed, and (ii) that the wound healing and repair process is responsible for lengthening some, but not all, soft tissues, even though they have not been cut at the beginning of the lengthening. Skin, nerve, muscle, and other components of the soft tissues of the leg have their own proper function, biological activity, and biomechanical properties, so that the relationship between tissue injury and functional impairment must differ among these tissues. Many of the lines of evidence cited here arose from animal studies conducted in fields other than limb lengthening. Very little information on limb lengthening in humans in terms of the mechanism of soft tissue lengthening, is available. Further studies are needed to elucidate the mechanism of tissue formation and to determine the optimal lengthening conditions for each soft tissue.
Summary and future directions The responses of soft tissues to limb lengthening or stretching are: decreased physiological slack, tissue migration from the adjacent region, stress relaxation (characteristic of viscoelastic materials), tissue formation, and tissue injuries. Optimal conditions for lengthening, that is, conditions under which new tissues are formed without injury, have yet to be established. However, some types of injury trigger events that lead to
Acknowledgments.
This report is based on research done in the D e p a r t m e n t of Orthopaedic Surgery of the University of T o k y o under chairperson Prof. T. Kurokawa. Joint researchers were R. Murashima, S. Hiraki, K. Tamai, and W. Ou.
References 1. Antonio J, Gonyea WJ. Skeletal muscle fiber hyperplasia. Med Sci Sports Exerc 1993;25:1333-45. 2. Austad ED, Pasyk KA, McClatcheyKD et al. Histomorphologic evaluation of guinea pig skin and soft tissue after controlled tissue expansion. Plast Reconstr Surg 1982;70:704--10. 3. Austad ED, Thomas SB, Pasyk KA. Tissue expansion: Dividend or loan? Plast Reconstr Surg 1986;78:63--7. 4. Brown IA. A scanning electron microscopestudy of the effects of uniaxial tension on human skin. Br J Dermatol 1973;89:383-93. 5. Carroll NC, Grant CG, Hudson R, et al. Experimental observation on the effects of leg lengthening by the Wagner method. Clin Orthop 1981;160:250-7. 6. Chuang T-Y, Chan R-C, Chin L-S, et al. Neuromuscular injury during limb lengthening: A longitudinal follow-upby rabbit tibial model. Arch Phys Med Rehabil 1995;76:467-70. 7. Czerwinski SM, Martin JM, Bechtel PJ. Modulation of IGF mRNA abundance during stretch-induced skeletal muscle hypertrophy and regression. J Appl Physiol 1994;76:2026-30. 8. Dix DJ, Eisenberg BR. Myosin mRNA accumulation and myofibrilogenesisat the myotendinousjunction of stretched muscle fibers. J Cell Biol 1990;111:1885-94. 9. Doi M. An experimental study on elongation injury of peripheral nerve. Electrophysiological and morphological changes (in Japanese). Nittekai-shi (J Jpn Soc Surg Hand) 1990;6:1041-52. 10. Galadi G, Comi G, Lozza L, et al. Peripheral nerve damage during limb lengthening. Neurophysiologyin five cases of bilateral tibial lengthening. J Bone Joint Surg Br 1990;72:121-4. 11. Goldspink G, Scutt A, Martindale J, et al. Stretch and force generation induce rapid hypertrophy and myosin isoform gene
K. N a k a m u r a et al.: Soft tissue r e s p o n s e s to limb l e n g t h e n i n g
197
switching in adult skeletal muscle. Biochem Soc Trans 1991; 19:368-73. 12. Haftek J. Stretch injury of peripheral nerve. Acute effects of stretching on rabbit nerve. J Bone Joint Surg Br 1970;52:354-65. 13. Hiraki S, Kurokawa T, Oonishi I, et al. Changes in skin length and area during limb lengthening (in Japanese). J Jpn Orthop Assoc 1990;64:$89. 14. Hirsowitz B, Lindenbaum E, Har-Shai Y. A skin-stretching device for the harnessing of the viscoelastic properties of skin. Plast Reconstr Surg 1993;92:260-70. 15. Holly RG, Barnett JG, Ashmore CR, et ak Stretch-induced growth in chicken wing muscles: A new model of stretch hypertrophy. Am J Physiol 1980;238:C62-71. 16. Ilizarov GA. The tensirn-stress effect on the genesis and growth of tissues: Part I. The influence of stability of fixation and softtissue preservation. Clin Orthop 1989;238:249-81. 17. Ilizarov GA. The tension-stress effect on the genesis and growth of tissues: Part II. The influence of the rate and frequency of distraction. Clin Orthop 1989;239:263-85. 18. Johnson PE, Kernahan DA, Bauer BS. Dermal and epidermal response to soft-tissue expansion in the pig. Plast Reconstr Surg 1988;81:390-5. 19. Johnson TM, Lowe L, Brown MD, et al. Histology and physiology of tissue expansion. J Dermatol Surg Oncol 1993;19:1074-8. 20. Kendall JP, Stokes IAF, O'Hara JP, et al. Tension and creep phenomena in peripheral nerve. Acta Orthop Scand 1979;50:7215. 21. Kennedy JM, Eisenberg BR, Reid SK, et al. Nascent muscle fiber appearance in overload chicken slow-tonic muscle. Am J Anat 1988;181:203-15. 22. Lee DY, Han TR, Choi IH, et al. Changes in somatosensoryevoked potentials in limb lengthening. An experimental study on rabbits' tibiae. Clin Orthop 1992;285:273-9. 23. Lee DY, Choi IH, Chung CY, et al. Effect of tibial lengthening on the gastrocnemius muscle. A histopathologic and morphometric study in rabbits. Acta Orthop Scand 1993;64:688-92. 24. Lindboe CF, Field TO, Steen H. Morphological changes in continuously stretched skeletal muscles in sheep. Eur J Appl Physiol 1985;54:184-90. 25. Lundborg G, Rydevik B. Effects of stretching the tibial nerve of the rabbit. A preliminary study of the intraneural circulation and the barrier function of the perineurium. J Bone Joint Surg Br 1973;55:390--401. 26. Matsushita T, Kurokawa T, Asai J. Continuous monitoring of tensile force during limb lengthening (in Japanese). J Jpn Orthop Assoc 1988;62:$993. 27. Murashima R, Kurokawa T, Nagano A, et al. Changes in motor nerve conduction velocity during leg lengthening (in Japanese). J Jpn Orthop Assoc 1993;67:$495. 28. Nagaoka T, Abe M, Kinoshita M, et al. Experimental study of the elongated muscle in limb lengthening (in Japanese). Chubuseisaishi (Centr Jpn J Orthop Surg Traumatol) 1992; 35:579-80. 29. Nakamura K, Kurokawa T, Matsushita T, et al. Prevention of equinus deformity during tibial lengthening. Comparison of passive stretching with an orthosis. Int Orthop (in press).
30. Ogata K, Naito M. Blood flow of peripheral nerve effects of distraction, stretching, and compression. J Hand Surg 1986;11:1014. 31. Osbaldeston NJ, Lee DM, Cox VM, et al. The temporal and cellular expression of c-los and c-jun in mechanically stimulated rabbit latissimus dorsi muscle. Biochem J 1995;308:465-71. 32. Paley D. Current techniques of limb lengthening. J Pediatr Orthop 1988;8:73-92. 33. Pasyk KA, Austad ED, McClatchey KD, et al. Electron microscopic evaluation of guinea pig skin and soft tissue "expanded" with a self-inflating silicone implant. Plast Reconstr Surg 1982;70:37-45. 34. Pasyk KA, Argenta LC, Hassett C. Quantitative analysis of the thickness of human skin and subcutaneous tissue following controlled expansion with a silicone implant. Plast Reconstr Surg 1988;81:516-23. 35. Russell B, Dix D J, Haller DL. Repair of injured skeletal muscle: A molecular approach. Med Sci Sports Exerc 1992;24:18993. 36, Rydevik BL, Kwan MK, Myers RR, et al. A n in vitro mechanical and histological study of acute stretching on rabbit tibial nerve. J Orthop Res 1990;8:694-701. 37. Simpson AHRW, Williams PE, Kyberd P, et al. The response of muscle to leg lengthening. J Bone Joint Surg Br 1995;7:6306. 38. Tamai K, Kurokawa T, Matsubara I. In situ observation of adjustment of sarcomere length in skeletal muscle under sustained stretch. J Jpn Orthop Assoc 1989;63:1558-63. 39. Taylor DC, Dalton JD, Seaber AV, et al. Viscoelastic properties of muscle-tendon units. The biomechanical effects of stretching. Am J Sports Meal 1990;18:300-9. 40. Timmenga EJF, Sehoorl R, Klopper PJ. Biomechanical and histomorphological changes in expanded rabbit skin. Br J Plast Surg 1990;43:101-6. 41. Ugaji Y, Shinoda K, Yoshimizu N, et al. Experimental studies on the stretch injury of peripheral nerve. Measurement of the local blood flow by electrochemically generated hydrogen (in Japanese). Nittekai-shi (J Jpn Soc Surg Hand) 1985;2:573-7. 42. Wall EJ, Kwan MK, Rydevik BL, et al. Stress relaxation of a peripheral nerve. J Hand Surg 1991;16A:859-63. 43. Wall E J, Massie JB, Kwan MK, et al. Experimental stretch neuropathy. Changes in nerve conduction under tension. J Bone Joint Surg Br 1992;74:126-9. 44. Williams PE, Goldspink G. The effect of immobilization on the longitudinal growth of striated muscle fibres. J Anat 1973;116:4555. 45. Williams PE, Goldspink G. Changes in sarcomere length and physiological properties in immobilized muscle. J Anat 1978;127:459~58. 46. Yasui N, Kojimoto H, Shimizu H, et al. The effect of distraction upon bone, muscle, and periosteum. Orthop Clin North Am 1991;22:563-7. 47. Yoshimoto H, Abe M, Doi M, et al. An experimental study of peripheral nerve elongation in limb lengthening (in Japanese). J Jpn Orthop Assoc 1992;66:$1462.
J Orthop Sci (1997) 2:191-197
]ournal of
thopaedic Science The Japanese Orthopaedic Association
Instructional lecture
Soft tissue responses to limb lengthening K o z o NAKAMURA, TAKASHI MATSUSHITA,HIROSHI OKAZAKI, and TAKAHIDE KUROKAWA Department of Orthopaedic Surgery, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Abstract: The responses of soft tissues to limb lengthening or stretching are: reduction in physiological slack, tissue migration from the adjacent region, stress relaxation (as occurs in viscoelastic materials), tissue formation, and tissue injuries. Theoretically, an optimal condition for limb lengthening is that under which new tissues are formed without injury. However, phenomena, such as angiogenesis in the dermis, due to tissue hypoxia, suggest that injury triggers tissue formation; the conditions under which tissue formation and tissue injury occur are not always opposed. Components of the soft tissues of the leg have their own distinctive functions, biological activity, and biomechanical properties, so that the relationship between tissue injury and functional impairment differs among these tissues. Very little information on limb lengthening in humans is available, in terms of the mechanism of soft tissue lengthening. Further work is needed to determine the optimal lengthening conditions for each soft tissue. Key words: limb lengthening, muscle, peripheral nerve, skin, stretch
Introduction
Limb lengthening is used to correct limb length discrepancy and to increase short stature. As limbs consist of bone and soft tissues, limb lengthening involves the lengthening of both these components. Bone response can be evaluated by various methods, for example, by X-ray and dual energy X-ray absorptio-metry. Many factors have been studied to establish clinical guidelines for bone lengthening and problems with bone formation can now be controlled well. 1~,17,32However, clinical methods for evaluating soft tissue responses to distrac-
Offprint requests to: K. Nakamura Received for publication on Nov. 21, 1996; accepted on Dec. 16, 1996
tion have not yet been established, and problems relating to these tissues often cause clinical complications such as joint contracture, deformity of the lengthened bone, and neurological complications. 32 In limb lengthening, bone is initially osteotomized and a type of repair process is responsible for bone formation during lengthening. Since soft tissues are not cut before lengthening begins, a biological process different from that in bone is thought to be involved. Skin, nerve, muscle, and other components of the soft tissues are viscoelastic, 14,2~ that is, they exhibit timedependent behavior. When the rate of loading increases, the modulus increases. When the tissue is held at a constant deformation, load decreases (stress relaxation). There is no doubt that this viscoelastic behavior is, at least in part, responsible for the lengthening of soft tissue. During lengthening, tensile force repeats a cycle of instantaneous increase after distraction, followed by reduction, mimicking stress relaxation. When lengthening was performed twice a day, 0.5 mm at 8 a.m. and 8 p.m., the reduction rate during the 12 night hours was significantly higher than that during the daytime (Fig. 1). 26 Since non-living viscoelastic material shows no difference within a period of i day, this difference is thought to reflect some biological response. One possibility is the growth of tissues.
Peripheral nerves
Responses to limb lengthening The magnitude of nerve elongation is not consistent with that of bone lengthening. In the rabbit, elongation of the femoral nerve (from the sciatic notch to the bifurcation of the peroneal nerve) for 10% and 20% femoral lengthening was 4.5 % and 11.7 %, respectively? Elongation of nerves occurs not only at the site of the
192
K. Nakamura et al.: Soft tissue responses to limb lengthening
k9
10
0 0
I 2
I 4
I 6
I 8
osteotomy but also throughout the nerves. In this animal, femoral nerve elongation occurred equally in the proximal, middle, and distal segments. 9 Nerves are highly viscoelastic tissues and show m a r k e d stress relaxation. The mean 1-h stress relaxation of rabbit tibial nerves stretched for 6%, 9%, and 12% of their original resting length was 48%, 34%, and 34%, respectively. 42 U n d e r constant elongation of rat sciatic nerve produced by 30-g tension, the tension was reduced by 30% in the first 10min and by a small amount in the ensuing 20 min. 2~ Increase of internodal length is reported to be one response to limb lengthening; internodal length increased by 11% in 18.5% femoral nerve elongation with femoral lengthening conducted at the rate of 0.5-ram twice a day. 47 Ilizarov 17 reported nerve fibers at various stages of formation and differentiation as evidence of new nerve formation during tibial lengthening in dogs of 0.25 mm every 6 h or less often. It has been pointed out that the frequency of peripheral nerve complications during limb lengthening is underestimated and that peripheral nerve dysfunction can be detected more frequently by electrophysiological tests: compound muscle action potential 6 and somatosensory-evoked potential 22 having been used in rabbit tibial lengthening, and electromyography and motor conduction velocity in human tibial lengthening? ~ The incidence of neurological dysfunction depended on the rate and frequency of distraction. Murashima et al. 27 measured the motor nerve conduction velocity (MCV) of the deep peroneal nerve during tibial lengthening in humans. A decrease in velocity (to less than 40m/s) was seen in 14 of 18 nerves at 3 0 % - 9 8 % lengthening done manually at the rate of more than 0.5 mm/ day in two steps per day. However, even when the limb was lengthened in two steps per day, there was no de-
i I Odays
Fig. 1. Time course of tensile force during limb lengthening.26 Femoral lengthening of 0.5 mm was performed twice a day, at 8 a.m., and 8 p.m., in a 12-year-old girl. Force was increased progressively, repeating a cycle of increase and reduction. Note that the reduction rate of the force during the 12 night hours was higher than that during the daytime. Circles, Morning; dots, evening
crease of MCV in nerves in which the mean distraction rate was less than 0.5mm per day. Further, there was no decrease of MCV in nerves in which distraction, at a frequency of 1440 steps per day, was produced by an autodistractor (Hifixator system; Nagano Keiki, Nagano, Japan), even if the limb was lengthened at the rate of 0.5-1.0mm/day. Ilizarov 17 reported that at a distraction rate of l mm once every 24h, nerve fibers exhibited uneven axon diameter and formation of irregular accumulations of cytoplasm, but at a distraction rate of 0.017mm every 24rain using his autodistractor, the nerve fibers had a normal structure. Major nerve injury complications also depend on the amount of lengthening; they rarely occur with lengthening of less than 10cm, but with lengthening of more than 10cm, 12% of patients developed transient sensory or motor loss. 32This close relationship between the amount of distraction and nerve dysfunction was confirmed in the rabbit tibial nerve during tibial lengthening. 22
Responses to acute stretching During acute stretching, within the physiological range of limb motion, the nerve may initially, stretch by up to 15% strain under minimal tension, much of the elongation being due to removal of the slack in the fascicles and in the nerve trunk. 36 Normal nerves have a wavy, sinusoidal shape in longitudinal section. On stretching, this sinusoidal shape is lost, 36the cross-sectional area of the funiculi is markedly reduced, and fibers become straight and closely packed. 12 The strain-stress curve (strain rate, 0.5%/s) of the rabbit tibial nerve showed low stiffness up to about 15%. The linear stiffness was reached at about 20% tensile strain and remained unchanged until failure. 36
K. Nakamura et al.: Soft tissue responses to limb lengthening For the rabbit tibial nerve, the ultimate strain was reported to be 39% at a strain rate of 0.5%/s, and 3673% at a strain rate of 0.5 mm/min 12, and the ultimate load was 7-10 N. 36 For human ulnar nerve, the ultimate load was 65-155 N? 6 There were multiple ruptures of perineurial sheaths when the nerve failed mechanically, indication that the perineurium may be a major load-carrying tissue component. 36 The importance of sufficient blood supply for nerve function is well known. 25As to changes in intraneural microcirculation due to acute stretching, a slowing down in venular flow was seen at 5 % - 1 0 % elongation of the rabbit tibial nerve, and complete cessation of arteriolar and capillary flow occurred at 11%-18% elongation. 25 Other authors have reported that 16% stretching caused complete arrest of blood flow in the rabbit sciatic nerve 3~ and the rabbit tibial nerve. 4~ The influence of elongation on the intraneural microvascular flow may be due to the reduced cross-sectional area, causing an increase in intrafascicular pressure and consequent interference with intrafascicular nutritive blood flow. 25 Nerve dysfunction was observed even when no morphological change was detectedY ,43 The functional changes in a stretched nerve are highly dependent on the magnitude and character of the deforming force, as well as on the length of time during which the force operates. As little as 6% strain caused a 70 % decrease in action potential amplitude if it lasted for 1 h in the rabbit tibial nerve; the onset of deficits may be delayed. 43 Since this injury occurred at a strain lower than that which produced intraneural arterial occlusion, 25 it appears that mechanical deformation, not ischemia, may be responsible for the early dysfunction.
193
Skin
Responses to limb lengthening Skin is the only tissue whose changes can be observed directly during lengthening. In human patients, Hiraki et al. 13drew a grid pattern, with lines perpendicular and lines parallel to the axis of the leg, on the skin surface from 15cm above the knee to 10cm below the ankle. They measured changes in length and area during tibial lengthening, using the Hifixator (Nagaro Keiki) a unilateral type lengthener. At 60% bone lengthening, the increases in length of the skin on the anterior surface were: 68% midway between the two clamps, 60% at the distal clamp, 35% at the proximal claknp, 13% at the foot, and 10% above the knee (Fig. 2). Skin migration into the expanded field was noted: 8.Smm in the proximal direction of the ankle skin and 8.0 mm in the distal direction of the knee skin at 60 % bone lengthening. 13 Ilizarov 16 described histological findings of hyperplasia of basal cells, hair follicles, and sweat and sebaceous glands after tibial lengthening in dogs as evidence of histogenesis due to lengthening.
Responses to tissue expansion Normal skin has slack in the resting position. The epidermis has a two-stage response to progressive strain. Initially, its undulating form is pulled out flat and further stretching elongates the cells. In response to increasing strain, the dermal multidirectional system of wavy fibers is reorientated, becoming aligned and compacted? The mechanism of increase in skin surface area after expansion can be explained, at least in part, in terms of
% 8060tD
b (=
O-20
Region
i
30
A
60
0
30
60
0
30
60
0
30
% bone lengthening B C D
60
0
30
E
60
Fig. 2. Changes in skin length on the anterior surface of the leg in each region during tibial lengthening.13 A grid pattern drawn on the skin surface was analyzed from 15 cm above the knee to 10 cm below the ankle. At 60% bone lengthening, the increases were: 68% midway between the clamps (C), 60% at the distal clamp (D), 35% at the proximal clamp (B), 13% at the foot (E), and 10% above the knee (A). (n - 10)
194
K. Nakamura et al.: Soft tissue responses to limb lengthening
stress relaxation, because the skin is viscoelastic. The oxygen saturation of the skin dropped steeply for a period of up to 20s and then rapidly returned to its initial normal value within as stress relaxation took place minutes. As the expansion was repeated, however, the oxygen saturation recovery time became slower. TM Capillaries are created in the papillary dermis after tissue expansion, and tissue hypoxia due to distraction may contribute to the neovascularization.19 The inflation of a tissue expander resulted in a threefold elevation of epidermal mitotic activity within 24 h, 3 and a significant thickening of the epidermis of pig skin. 2,18,19,34On the other hand, the dermis and subcutaneous tissue were significantly thinner after expansion in humans 34and pigs. 18 With a chronic stretching force, fibroblasts in the dermis increased in number, and the rough endoplasmic reticulum of these cells contained large cisternae, indicating increased metabolic activity.33 Although dermis thinning persisted for 36 weeks after expansion in pig skin, the total collagen content, calculated within an expanded square grid, increased; the increase in surface area was not merely due to stretching of the skin but to new tissue having been created in the dermal layer because of the chronic stretching force. TM However, since the dermal thinning persisted, it seems that compensation is slower in this layer, in contrast to the rapid compensation seen in the epidermal layer. Although the gross dermal architecture remained unchanged, the collagen bundles in the expanded skin showed an increased diameter, with loosely packed collagen fibrils. In terms of biomechanical properties, the
expanded skin showed 60% less stiffness than before expansion, and deformation between maximal stiffness and failure increased by 20%. These findings indicate damage to the dermis. Therefore, the new tissue formation that takes place in the dermis because of expansion may be initiated by wound healing and repair. The normalization of the dermis 2 years after expansion reported by Pasyk et al. 34 seems to confirm this sequence of events. 4~
Muscle
Responses to limb lengthening Gain in length of the musculotendinous unit can be determined by evaluating the range of motion of the adjacent joint. The dorsiflexion angle of the ankle joint was maintained even at 60% tibial lengthening in patients in whom orthosis was used to maintain the dorsiflexion of the ankle for a period of more than 16h/ day; there was an actual gain in length of triceps surae due to tibial lengthening (Fig. 3). 29During tibial lengthening in rabbits (0.5mm, twice daily for 20 days) the weight of the tibialis anterior muscle increased significantly more on the lengthened side than on the contralateral side. 28 During distraction, the muscles lengthen throughout the entire muscle, and not simply at the level of the osteotomy.46A gain in length has also been observed in the tendinous portion, as well as in the muscle belly; with 2-cm lengthening, the former was 0.67 cm and the latter 1.22 cm.28Since musculotendinous
degree I
I
30 !
I
r-"l
i
20
!
I
I
I-'-1
!*i
N'8
10" tO X
N-8 N-6 N-10
0 -10
N'6 N-5
0 "0 tt~
N-E
-20
N-6
N-8
-30 -40
N-8 N-12
-50
-
10
20
30 % length gain
40
50
%
Fig. 3. Relationship between dorsiflexion angle of the ankle and percent tibial lengthening. Comparison of effectiveness of three methods in preventing an equinus deformity during tibial lengthening.29Circles, Orthosis _->16h/day; squares, orthosis <16 h/day; triangles, physiotherapy. *P < 0.05; **P < 0.01
K. Nakamura et al.: Soft tissue responses to limb lengthening
195
units respond viscoelastically to tensile loads, 39 stress relaxation must be one of the factors responsible for the increase in length. The results of previous studies vary regard in to the findings on the influence of limb lengthening on muscular fiber type: reports show a shift from type 1 to type 2 (sheep), 5 no change in the proportion of muscle fiber types (rabbit, gastrocnemius), 23 decrease in mean size of type 1 and 2 muscle fibers (rabbit, gastrocnemius), 23 type 1 fiber hypertrophy (rabbit, tibialis anterior), and 28 type 2 fiber atrophy (Sheep, biceps femoris). 24 These results may indicate the influence of muscular inactivity during limb lengthening, as well as the influence of continuous stretching. Muscle damage, such as internalization of nuclei and endomysial fibrosis, has also been reported. The severity of histological changes was reported to be related linearly to the percentage of lengthening; substantial changes occurred in groups subjected to 20% and 30% bone lengthening (rabbit, gastrocnemius), 23 or to lengthening of more than 11% (sheep, tibial lengthening). 5 Muscle damage also depended on the rate of distraction, so that it was seen in all tibialis anterior muscles of the rabbit when they were lengthened at a rate of 1 mm or more. Limbs distracted at higher rates tended to show dysfunction even if the amount of lengthening was the same. 37
creased immediately after stretching, gradually normalized within 3 weeks 45or 7 days (Fig. 4) 38 in mice. The n u m b e r of sarcomeres increased within 2 weeks in the mouse soleus muscle. 44 These findings suggest that the stretched muscles are lengthened by the addition of new sarcomeres. A t the myotendinous junction (MTJ) of tibialis anterior muscles stretched for 4 days, there were higher levels of myosin heavy chain (MHC) m R N A , greatly increased amounts of vinculin (a major component of myofibril attachment at the MTJ), and a large cytoplasmic space containing nascent filament assemblies. 8 These results support the idea that stretched muscles lengthen by adding sarcomeres at the MTJ. Other findings showed nascent fibers and myotubes at the ends of the muscles after 6 days of stretching. It is thought that these myotubes fuse with and become extensions of existing fibers, similar to observations during development. 8 The time course of changes in muscle cross-sectional area was different from that in length, suggesting that the underlying molecular events responsible for longitudinal and cross-sectional growth may be independent. 15 Stretch-induced enlargement of the muscles is due to both an increase in fiber area (hypertrophy) and to an increase in fiber number (hyperplasia). 1 Satellite cells are postulated to be the sole source of myonuclei, and adult muscles can increase their mass or repair an injury by recruiting satellite cells, which then fuse with existing fibers (hypertrophy) or fuse together to form new myotubes (hyperplasia). 21,35After injury, in the rat, the satellite cells take several hours to become mobilized, and cell division is highest 2-4 days later. 35 In the stretched muscle fibers, myosin m R N A is concentrated around the nucleus and in the region corresponding to disruption of the normal myofibrillar architecture, providing local contractile protein synthesis for myofibril
Responses to immobilization in the stretched position The responses of striated muscles to stretching have been studied in models in which the muscles were maintained in the stretched position by casting of the limb. Increases in length and cross-sectional area and changes in fiber type have been reported. When the muscle was stretched and held in the stretched position, the sarcomere length, which was in-
E v~
3.0-
I-7u J
2.9-
'"
2.8"
~
(9)
(10) * A
E E
0
'~
2.7-
~(8)
~(10) t(10)
~(10) ~(7)
fi "4 1.2
1.0
Control
;
;
.
5
.
.
pl i
.
7
IMMOBILIZATION PERIOD (days)
14
"I" I-(.D ZI,M
Fig. 4. Time course of changes in sarcomere length (dots) and muscle length (triangles) of the extensor digitorum longus muscle during sustained stretching.38The limbs of mice were immobilized with the ankle and toes in full plantar flexion with a plaster cast. Sarcomere length was measured in situ by the light diffraction method. The sarcomere lengthened immediately after stretching and normalized gradually by 7 days. The muscle length showed no significant change during the period of immobilization. Values are means _+ SEM. *P < 0.05; **P < 0.01. Numbers in parentheses indicate numbers of animals
196
K. Nakamura et al.: Soft tissue responses to limb lengthening
assembly and repair. 35 The repeated injury of muscle fibers, followed by regeneration, may produce an overcompensation of protein synthesis, resulting in a net anabolic effect. 1 G r o w t h factors appear to be involved in the activation of satellite cell proliferation and availability for the repair process. Fibroblast growth factor (FGF) stimulates cell proliferation, whereas transforming growth factor-~ (TGF-~) has the opposite effect, supressing the proliferation of cultured satellite cells in vitro. Insulinlike growth factor (IGF) overrides the effects of TGF-~ and stimulates the proliferation of satellite cellsY Czerwinski e t al. 7 showed that, after 11 days of stretching the patagialis muscle of the chicken, the muscle weight increased by 60% and IGF-I m R N A levels increased threefold, suggesting a role for IGF-I in muscle hypertrophy. In striated muscle, a variety of mechanical stimuli have been shown to lead to a rapid and transient increase in the expression of a number of cellular oncogenes. A 15% stretching of the rabbit latissimus dorsi muscle caused peak expression of c-fos and c-jun m R N A 1 h after the imposition of stretching, determined by reverse transcription polynerase chain reaction. Immunostaining with anti-Fos and anti-Jun antibodies revealed the accumulation of these proteins in both myofiber and interstitial cell nuclei after passive stretching. These findings suggest that rapid induction of c-fos and c-jun is an early event in response to mechanical stretching and may trigger events leading to muscle fiber hypertrophy. 31 Slow-contracting and fast-contracting fibers are characterized by different protein isoforms, and the differentiation into fast or slow type fibers involves regulation of the expression of different subsets of genes. Stretching of the tibialis anterior muscle in an adult rabbit caused gene switching, which involved repression of a fast and activation of a slow myosin heavy chain gene, together with rapid hypertrophy within a period as short as 4 days. 11 This switching seems to be a reasonable adaptation, as the slow myosin heavy chain has lower specific ATPase activity and is therefore more economical for maintaining isometric force.
tissue formation, for example, angiogenesis in the dermis due to tissue hypoxia, dermis formation after tissue expansion, and muscle hypertrophy or hyperplasia that occurs via the recruitment of satellite cells. These p h e n o m e n a indicate: (i) that conditions for tissue formation and injury are not always opposed, and (ii) that the wound healing and repair process is responsible for lengthening some, but not all, soft tissues, even though they have not been cut at the beginning of the lengthening. Skin, nerve, muscle, and other components of the soft tissues of the leg have their own proper function, biological activity, and biomechanical properties, so that the relationship between tissue injury and functional impairment must differ among these tissues. Many of the lines of evidence cited here arose from animal studies conducted in fields other than limb lengthening. Very little information on limb lengthening in humans in terms of the mechanism of soft tissue lengthening, is available. Further studies are needed to elucidate the mechanism of tissue formation and to determine the optimal lengthening conditions for each soft tissue.
Summary and future directions The responses of soft tissues to limb lengthening or stretching are: decreased physiological slack, tissue migration from the adjacent region, stress relaxation (characteristic of viscoelastic materials), tissue formation, and tissue injuries. Optimal conditions for lengthening, that is, conditions under which new tissues are formed without injury, have yet to be established. However, some types of injury trigger events that lead to
Acknowledgments.
This report is based on research done in the D e p a r t m e n t of Orthopaedic Surgery of the University of T o k y o under chairperson Prof. T. Kurokawa. Joint researchers were R. Murashima, S. Hiraki, K. Tamai, and W. Ou.
References 1. Antonio J, Gonyea WJ. Skeletal muscle fiber hyperplasia. Med Sci Sports Exerc 1993;25:1333-45. 2. Austad ED, Pasyk KA, McClatcheyKD et al. Histomorphologic evaluation of guinea pig skin and soft tissue after controlled tissue expansion. Plast Reconstr Surg 1982;70:704--10. 3. Austad ED, Thomas SB, Pasyk KA. Tissue expansion: Dividend or loan? Plast Reconstr Surg 1986;78:63--7. 4. Brown IA. A scanning electron microscopestudy of the effects of uniaxial tension on human skin. Br J Dermatol 1973;89:383-93. 5. Carroll NC, Grant CG, Hudson R, et al. Experimental observation on the effects of leg lengthening by the Wagner method. Clin Orthop 1981;160:250-7. 6. Chuang T-Y, Chan R-C, Chin L-S, et al. Neuromuscular injury during limb lengthening: A longitudinal follow-upby rabbit tibial model. Arch Phys Med Rehabil 1995;76:467-70. 7. Czerwinski SM, Martin JM, Bechtel PJ. Modulation of IGF mRNA abundance during stretch-induced skeletal muscle hypertrophy and regression. J Appl Physiol 1994;76:2026-30. 8. Dix DJ, Eisenberg BR. Myosin mRNA accumulation and myofibrilogenesisat the myotendinousjunction of stretched muscle fibers. J Cell Biol 1990;111:1885-94. 9. Doi M. An experimental study on elongation injury of peripheral nerve. Electrophysiological and morphological changes (in Japanese). Nittekai-shi (J Jpn Soc Surg Hand) 1990;6:1041-52. 10. Galadi G, Comi G, Lozza L, et al. Peripheral nerve damage during limb lengthening. Neurophysiologyin five cases of bilateral tibial lengthening. J Bone Joint Surg Br 1990;72:121-4. 11. Goldspink G, Scutt A, Martindale J, et al. Stretch and force generation induce rapid hypertrophy and myosin isoform gene
K. N a k a m u r a et al.: Soft tissue r e s p o n s e s to limb l e n g t h e n i n g
197
switching in adult skeletal muscle. Biochem Soc Trans 1991; 19:368-73. 12. Haftek J. Stretch injury of peripheral nerve. Acute effects of stretching on rabbit nerve. J Bone Joint Surg Br 1970;52:354-65. 13. Hiraki S, Kurokawa T, Oonishi I, et al. Changes in skin length and area during limb lengthening (in Japanese). J Jpn Orthop Assoc 1990;64:$89. 14. Hirsowitz B, Lindenbaum E, Har-Shai Y. A skin-stretching device for the harnessing of the viscoelastic properties of skin. Plast Reconstr Surg 1993;92:260-70. 15. Holly RG, Barnett JG, Ashmore CR, et ak Stretch-induced growth in chicken wing muscles: A new model of stretch hypertrophy. Am J Physiol 1980;238:C62-71. 16. Ilizarov GA. The tensirn-stress effect on the genesis and growth of tissues: Part I. The influence of stability of fixation and softtissue preservation. Clin Orthop 1989;238:249-81. 17. Ilizarov GA. The tension-stress effect on the genesis and growth of tissues: Part II. The influence of the rate and frequency of distraction. Clin Orthop 1989;239:263-85. 18. Johnson PE, Kernahan DA, Bauer BS. Dermal and epidermal response to soft-tissue expansion in the pig. Plast Reconstr Surg 1988;81:390-5. 19. Johnson TM, Lowe L, Brown MD, et al. Histology and physiology of tissue expansion. J Dermatol Surg Oncol 1993;19:1074-8. 20. Kendall JP, Stokes IAF, O'Hara JP, et al. Tension and creep phenomena in peripheral nerve. Acta Orthop Scand 1979;50:7215. 21. Kennedy JM, Eisenberg BR, Reid SK, et al. Nascent muscle fiber appearance in overload chicken slow-tonic muscle. Am J Anat 1988;181:203-15. 22. Lee DY, Han TR, Choi IH, et al. Changes in somatosensoryevoked potentials in limb lengthening. An experimental study on rabbits' tibiae. Clin Orthop 1992;285:273-9. 23. Lee DY, Choi IH, Chung CY, et al. Effect of tibial lengthening on the gastrocnemius muscle. A histopathologic and morphometric study in rabbits. Acta Orthop Scand 1993;64:688-92. 24. Lindboe CF, Field TO, Steen H. Morphological changes in continuously stretched skeletal muscles in sheep. Eur J Appl Physiol 1985;54:184-90. 25. Lundborg G, Rydevik B. Effects of stretching the tibial nerve of the rabbit. A preliminary study of the intraneural circulation and the barrier function of the perineurium. J Bone Joint Surg Br 1973;55:390--401. 26. Matsushita T, Kurokawa T, Asai J. Continuous monitoring of tensile force during limb lengthening (in Japanese). J Jpn Orthop Assoc 1988;62:$993. 27. Murashima R, Kurokawa T, Nagano A, et al. Changes in motor nerve conduction velocity during leg lengthening (in Japanese). J Jpn Orthop Assoc 1993;67:$495. 28. Nagaoka T, Abe M, Kinoshita M, et al. Experimental study of the elongated muscle in limb lengthening (in Japanese). Chubuseisaishi (Centr Jpn J Orthop Surg Traumatol) 1992; 35:579-80. 29. Nakamura K, Kurokawa T, Matsushita T, et al. Prevention of equinus deformity during tibial lengthening. Comparison of passive stretching with an orthosis. Int Orthop (in press).
30. Ogata K, Naito M. Blood flow of peripheral nerve effects of distraction, stretching, and compression. J Hand Surg 1986;11:1014. 31. Osbaldeston NJ, Lee DM, Cox VM, et al. The temporal and cellular expression of c-los and c-jun in mechanically stimulated rabbit latissimus dorsi muscle. Biochem J 1995;308:465-71. 32. Paley D. Current techniques of limb lengthening. J Pediatr Orthop 1988;8:73-92. 33. Pasyk KA, Austad ED, McClatchey KD, et al. Electron microscopic evaluation of guinea pig skin and soft tissue "expanded" with a self-inflating silicone implant. Plast Reconstr Surg 1982;70:37-45. 34. Pasyk KA, Argenta LC, Hassett C. Quantitative analysis of the thickness of human skin and subcutaneous tissue following controlled expansion with a silicone implant. Plast Reconstr Surg 1988;81:516-23. 35. Russell B, Dix D J, Haller DL. Repair of injured skeletal muscle: A molecular approach. Med Sci Sports Exerc 1992;24:18993. 36, Rydevik BL, Kwan MK, Myers RR, et al. A n in vitro mechanical and histological study of acute stretching on rabbit tibial nerve. J Orthop Res 1990;8:694-701. 37. Simpson AHRW, Williams PE, Kyberd P, et al. The response of muscle to leg lengthening. J Bone Joint Surg Br 1995;7:6306. 38. Tamai K, Kurokawa T, Matsubara I. In situ observation of adjustment of sarcomere length in skeletal muscle under sustained stretch. J Jpn Orthop Assoc 1989;63:1558-63. 39. Taylor DC, Dalton JD, Seaber AV, et al. Viscoelastic properties of muscle-tendon units. The biomechanical effects of stretching. Am J Sports Meal 1990;18:300-9. 40. Timmenga EJF, Sehoorl R, Klopper PJ. Biomechanical and histomorphological changes in expanded rabbit skin. Br J Plast Surg 1990;43:101-6. 41. Ugaji Y, Shinoda K, Yoshimizu N, et al. Experimental studies on the stretch injury of peripheral nerve. Measurement of the local blood flow by electrochemically generated hydrogen (in Japanese). Nittekai-shi (J Jpn Soc Surg Hand) 1985;2:573-7. 42. Wall EJ, Kwan MK, Rydevik BL, et al. Stress relaxation of a peripheral nerve. J Hand Surg 1991;16A:859-63. 43. Wall E J, Massie JB, Kwan MK, et al. Experimental stretch neuropathy. Changes in nerve conduction under tension. J Bone Joint Surg Br 1992;74:126-9. 44. Williams PE, Goldspink G. The effect of immobilization on the longitudinal growth of striated muscle fibres. J Anat 1973;116:4555. 45. Williams PE, Goldspink G. Changes in sarcomere length and physiological properties in immobilized muscle. J Anat 1978;127:459~58. 46. Yasui N, Kojimoto H, Shimizu H, et al. The effect of distraction upon bone, muscle, and periosteum. Orthop Clin North Am 1991;22:563-7. 47. Yoshimoto H, Abe M, Doi M, et al. An experimental study of peripheral nerve elongation in limb lengthening (in Japanese). J Jpn Orthop Assoc 1992;66:$1462.