Muscle regeneration and fiber-type transformation during distraction osteogenesis

Mechanisms governing protective pregnancy-induced adaptations of the pelvic floor muscles in the rat preclinical model

BACKGROUND

The intrinsic properties of pelvic soft tissues in women who do and do not sustain birth injuries are likely divergent. However, little is known about this. Rat pelvic floor muscles undergo protective pregnancy-induced structural adaptations-sarcomerogenesis and increase in intramuscular collagen content-that protect against birth injury.

OBJECTIVE

We aimed to test the following hypotheses: (1) the increased mechanical load of a gravid uterus drives antepartum adaptations; (2) load-induced changes are sufficient to protect pelvic muscles from birth injury.

STUDY DESIGN

The independent effects of load uncoupled from the hormonal milieu of pregnancy were tested in 3- to 4-month-old Sprague-Dawley rats randomly divided into the following 4 groups, with N of 5 to 14 per group: (1) load-/pregnancy hormones- (controls), (2) load+/pregnancy hormones-, (3) reduced load/pregnancy hormones+, and (4) load+/pregnancy hormones+. Mechanical load of a gravid uterus was simulated by weighing uterine horns with beads similar to fetal rat size and weight. A reduced load was achieved by unilateral pregnancy after unilateral uterine horn ligation. To assess the acute and chronic phases required for sarcomerogenesis, the rats were sacrificed at 4 hours or 21 days after bead loading. The coccygeus, iliocaudalis, pubocaudalis, and nonpelvic tibialis anterior musles were harvested for myofiber and sarcomere length measurements. The intramuscular collagen content was assessed using a hydroxyproline assay. An additional 20 load+/pregnancy hormones- rats underwent vaginal distention to determine whether the load-induced changes are sufficient to protect from mechanical muscle injury in response to parturition-associated strains of various magnitude. The data, compared using 2-way repeated measures analysis of variance followed by pairwise comparisons, are presented as mean±standard error of mean.

RESULTS

An acute increase in load resulted in significant pelvic floor muscle stretch, accompanied by an acute increase in sarcomere length compared with nonloaded control muscles (coccygeus: 2.69±0.03 vs 2.30±0.06 μm, respectively, P<.001; pubocaudalis: 2.71±0.04 vs 2.25±0.03 μm, respectively, P<.0001; and iliocaudalis: 2.80±0.06 vs 2.35±0.04 μm, respectively, P<.0001). After 21 days of sustained load, the sarcomeres returned to operational length in all pelvic muscles (P>.05). However, the myofibers remained significantly longer in the load+/pregnancy hormones- than the load-/pregnancy hormones- in coccygeus (13.33±0.94 vs 9.97±0.26 mm, respectively, P<.0001) and pubocaudalis (21.20±0.52 vs 19.52±0.34 mm, respectively, P<.04) and not different from load+/pregnancy hormones+ (12.82±0.30 and 22.53±0.32 mm, respectively, P>.1), indicating that sustained load-induced sarcomerogenesis in these muscles. The intramuscular collagen content in the load+/pregnancy hormones- group was significantly greater relative to the controls in coccygeus (6.55±0.85 vs 3.11±0.47 μg/mg, respectively, P<.001) and pubocaudalis (5.93±0.79 vs 3.46±0.52 μg/mg, respectively, P<.05) and not different from load+/pregnancy hormones+ (7.45±0.65 and 6.05±0.62 μg/mg, respectively, P>.5). The iliocaudalis required both mechanical and endocrine cues for sarcomerogenesis. The tibialis anterior was not affected by mechanical or endocrine alterations. Despite an equivalent extent of adaptations, load-induced changes were only partially protective against sarcomere hyperelongation.

CONCLUSION

Load induces plasticity of the intrinsic pelvic floor muscle components, which renders protection against mechanical birth injury. The protective effect, which varies between the individual muscles and strain magnitudes, is further augmented by the presence of pregnancy hormones. Maximizing the impact of mechanical load on the pelvic floor muscles during pregnancy, such as with specialized pelvic floor muscle stretching regimens, is a potentially actionable target for augmenting pregnancy-induced adaptations to decrease birth injury in women who may otherwise have incomplete antepartum muscle adaptations.

Clubfoot pathology in fetus and pathogenesis. A new pathogenetic theory based on pathology, imaging findings and biomechanics-a narrative review

Several studies have described the pathology of idiopathic congenital clubfoot (ICCF) in fetus. Numerous pathogenetic theories have been postulated on ICCF, but many of them lack any objective evidence. Pathologic studies in fetus together with MRI studies in patients with ICCF seem to favor the theory of a muscular imbalance of the foot activators during fetal growth as the main pathogenetic factor of ICCF. Our objectives were: (I) To support the theory of muscular imbalance as the primary pathogenetic factor of ICCF; (II) To clarify why atrophy and shortening affect the activator muscles of the foot unevenly, as reported by literature. A literature search based on MEDLINE and the COCHRANE database was performed to identify all published studies from 1929 to 2020 which report ICCF pathology in fetus, its etiopathogenesis, and imaging and biomechanical studies showing how the basic pathology may be addressed by Ponseti treatment. A manual search was also performed of the references cited in studies, reviews, and university libraries. Altered size, shape and articular relationships of the tarsal bones, and uneven atrophy and shortening of the leg muscles together with capsule and ligament abnormalities were the main pathologic findings reported in fetus with ICCF. Regarding ICCF pathogenesis, the main debate is between the advocators of a primitive blastemal defect of the tarsal bones leading to the skeletal abnormalities and those who hold that the latter are secondary to a deforming force generated by the soft tissues. Imaging studies have shown that the Ponseti method is able to address the skeletal abnormalities, the correction of which is maintained until adulthood, whereas leg muscle atrophy is not improved but tends to worsen with growth. Preliminary histochemical studies of the soleus-Achilles tendon junction have shown a decrease of the growth factors and the presence of myostatin, both down-regulators of muscle growth in patients with ICCF. The authors postulate that a defect of both the radial and the longitudinal growth unevenly affecting the leg muscles with a consequent imbalance of the foot activators might be the main pathogenetic factor of ICCF. Further studies are needed to confirm this theory.

Combined Technique for the Treatment of Infected Nonunions of the Distal Femur With Bone Loss: Short Supracondylar Nail-Augmented Acute Shortening/Lengthening

OBJECTIVE

To evaluate a combined technique for treating distal femoral bone defects after debridement of osteomyelitis, using an external fixator together with a short supracondylar nail.

DESIGN

Retrospective study.

SETTING

Single tertiary referral center.

METHODS

Between 2003 and 2018, 23 patients with a mean age of 37.2 years (26-56) underwent surgery with the same technique to manage postdebridement defects in the distal femur due to osteomyelitis. This involved acute shortening and intramedullary fixation of the defect site, together with relengthening from a proximal osteotomy using simultaneous external fixation. Radiographic union, range of motion of the hip and knee, external fixation time and external fixation index, and limb length discrepancy were assessed.

RESULTS

The mean follow-up was 51 months (18-192). Union was achieved in all patients without recurrence of infection during this follow-up period. The mean knee flexion was 120 degrees, and the mean extension deficit was 5 degrees at final follow-up. The mean limb length discrepancy improved from 5.5 cm (3-7) to 0.5 cm (0-2). The mean external fixation index was 29.2 d/cm (20-50), and the mean external fixation time was 115 days (90-150). Radiographic scores were excellent in 15 cases, good in 6, and fair in 2. Functional scores were excellent in 14 cases, good in 7, and fair in 2.

CONCLUSION

This combined strategy was an effective method for treating distal femoral segmental bone defects after debridement of osteomyelitis, with a high rate of union and acceptable complication rates.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Is Botulinum Toxin Type A a Valuable Adjunct During Femoral Lengthening? A Randomized Trial

BACKGROUND

Reduced joint ROM and distraction-induced pain are common complaints of patients who have undergone gradual femoral lengthening. Attempts to reduce the effects of lengthening on joint motion have included the use of botulinum toxin to reduce the muscle forces that restrict motion. The benefits of this approach during femoral lengthening, however, have not been conclusively established.

QUESTIONS/PURPOSES

We wished to evaluate the effects of botulinum toxin type A (BtX-A) injection in the anterior thigh muscles during femoral distraction osteogenesis on adjacent joint ROM and distraction-induced pain. We asked: (1) Does injection of BtX-A in the quadriceps muscles lead to improved knee and hip motion during femoral lengthening? (2) Does injection of BtX-A reduce pain during femoral lengthening?

METHODS

A single-center, double-blind, randomized placebo-controlled trial was conducted. Forty-four patients (88 femurs) undergoing bilateral femoral lengthening for familial short stature were included in the study. BtX-A (200 IU) was injected intraoperatively in the quadriceps muscles of one thigh. An equal volume of sterile normal saline was injected in the other thigh as a control. Selection of the limb receiving the toxin was randomized. Clinical evaluation included a VAS score for pain measurement, ROM evaluation of the hips and knees, and measurement of thigh circumference. Side-to-side differences were analyzed throughout the entire consolidation phase. No patients were lost to followup, leaving 44 patients (88 femurs). The mean followup was 26 months (range, 14-40 months). The distraction rate and final length of gain were similar between treated and control limbs. A priori power analysis suggested that 44 legs were required in each group to achieve statistical significance of 0.05 with 90% power to detect a 50% difference in treatment effect between treatment and control groups.

RESULTS

There were no differences in hip ROM, knee ROM, or maximal thigh circumference between the two lower extremities at any time during the study period. VAS scores were no different between the patients who received BtX-A and those who received saline.

CONCLUSIONS

Local injection of 200 IU BtX-A in the quadriceps muscles does not appear to reduce distraction-induced pain nor enhance ROM in the hip or knee during femoral lengthening. Additional studies are needed to evaluate the effect of larger doses or different injection methods. Based on our findings, we do not recommend routine use of botulinum injections during limb lengthening and believe any further use of this drug should only be in the context of a controlled trial.

LEVEL OF EVIDENCE

Level II, therapeutic study.

Dentate transport discs can be used to reconstruct large segmental mandibular defects

PURPOSE

This study tested the use of a dentate transport segment for the reconstruction of a large U-shaped defect in the anterior segment of the canine mandible using a novel curved reconstruction plate. The quality and quantity of bone regenerate formed by dentate versus edentulous transport segments were compared.

MATERIALS AND METHODS

In 5 adult foxhound dogs, a defect of 70 to 75 mm was created in the canine mandible by excising the mandible anterior to the right and left fourth premolars. Reconstruction was performed by trifocal distraction osteogenesis using a bone transport reconstruction plate (BTRP-02), with 2 transport units being activated simultaneously, one on either side of the defect, 1 dentate and 1 edentulous. Bilateral distraction proceeded at a rate of 1 mm/day until the segments docked against each other in the midline. After 39 to 44 days of consolidation, the animals were euthanized. The quantity and quality of bone regeneration on the 2 sides were compared using micro-computed tomography.

RESULTS

The defect reconstruction was successful. The amount and quality of bone formed by the transport segments were similar on the 2 sides. There were no major differences in the bone volume fraction and density of the regenerate bone formed by the 2 transport segments. The bone volume fraction and density of the regenerate bone were considerably lower than those of the host bone in the distal segments, likely owing to the short consolidation period.

CONCLUSIONS

Bone transport remains a viable option in reconstructing anterior segmental defects in the mandible. The use of dentate or edentulous transport segments for reconstruction provides options for the surgeon in often highly compromised patients requiring these surgeries.

Botulinum toxin a does not decrease calf pain or improve ROM during limb lengthening: a randomized trial

BACKGROUND

During lower limb lengthening, distraction-induced muscle pain and surrounding joint contractures are frustrating complications for which few effective treatments are available.

QUESTIONS/PURPOSES

We evaluated Botulinum Toxin Type A (BtX-A) injection in the calf muscles during human tibial distraction osteogenesis. We hypothesized that it may decrease calf pain and increase ROM of the surrounding joints by reducing muscle stiffness.

METHODS

Between April 2010 and January 2011, we evaluated 36 patients undergoing bilateral tibia lengthening who met prespecified inclusion criteria. All patients underwent stature lengthening with lengthening over a nail or lengthening and then nailing. BtX-A (200 IU) was injected at the calf muscle only in one leg for each patient and the same amount of sterile normal saline was injected into the other leg as a control. Selection of the leg receiving the toxin was randomized. Clinical evaluation included a VAS score for calf pain and measurement of ROM of the knees and ankles and calf circumference, with evaluations performed in a double-blinded manner. Side-to-side differences were analyzed until the end of consolidation phase. Minimum followup was 24 months (mean, 30 months; range, 24-39 months). The distraction rate and the final length gain were similar in the treated and control limbs. A priori power analysis suggested that 34 legs were required to achieve statistical significance of 0.05 with 80% of power to detect a 50% difference in treatment effect between treatment and control groups.

RESULTS

There were no differences in calf pain, knee and ankle ROM, and maximal calf circumferences between the two legs at each time point.

CONCLUSIONS

Local injection of 200 IU BtX-A at the human calf muscle does not appear to reduce calf pain or help enhance ROM of the knee and ankle during tibial lengthening. However, the small sample size provided sufficient power to detect only relatively large clinical effects; future, larger trials will be needed to determine whether smaller differences are present.

LEVEL OF EVIDENCE

Level II, therapeutic study. See Instructions for Authors for a complete description of levels of evidence.

Bone marrow aspirate concentrate and platelet-rich plasma enhanced bone healing in distraction osteogenesis of the tibia

BACKGROUND

During lower limb lengthening, poor bone regeneration is a devastating complication. Several local or systemic applications have been used to promote osteogenesis, and biologic stimulations are gaining attention, but their utility has not been proven in this setting.

QUESTIONS/PURPOSES

In patients undergoing bilateral tibial lengthening, we compared those receiving an osteotomy site injection of autologous bone marrow aspirate concentrate (BMAC) plus platelet-rich plasma (PRP) with those not receiving such an injection in terms of external fixator index (time in external fixation divided by amount of lengthening), full weightbearing index (time until a patient was permitted to do full weightbearing divided by amount of lengthening), four cortical healing indexes (time until each cortical union divided by amount of lengthening), and callus shape and type.

METHODS

Twenty-two patients (44 tibias) undergoing bilateral tibial lengthening enrolled in this randomized trial. Two patients were excluded, one due to insufficient radiographic evaluation and one who was lost to followup, leaving 20 patients (40 segments) for inclusion. Ten patients (20 segments) received BMAC combined with PRP injection (treatment group) and 10 patients (20 segments) received no injection (control group). All patients underwent stature lengthening for familial short stature with the lengthening over nail technique. Autologous BMAC combined with PRP was injected at the tibial osteotomy site at the end of the index surgery. Mean distraction rates were similar between groups (0.75 mm/day in the treatment group versus 0.72 mm/day in the control group; p = 0.24). Full weightbearing was permitted when we observed radiographic evidence of healing at two cortices; this assessment was made by the surgeon who was blinded to the treatment each patient received. Minimum followup was 24 months (mean, 28 months; range, 24-34 months).

RESULTS

There was no difference in mean external fixator index between groups. However, mean cortical healing indexes (anterior/posterior/medial/lateral) were 1.14/0.81/0.96/0.88 months/cm in the treatment group and 1.47/1.26/1.42/1.22 months/cm in the control group (all p < 0.001), showing faster healing in the treatment group at each cortex. Full weightbearing was permitted earlier in the treatment group than in the control group (index: 0.99 months/cm and 1.38 months/cm, respectively, p < 0.001). Callus shape and type were not different between groups.

CONCLUSIONS

Autologous BMAC combined with PRP injection at the osteotomy site helped improve bone healing in distraction osteogenesis of the tibia, although the effect size was small.

LEVEL OF EVIDENCE

Level I, therapeutic study. See Instructions for Authors for a complete description of levels of evidence.

Effects of hyperbaric oxygen therapy on biochemical and histological parameters of muscle groups in proximity to the distracted rat tibia

We investigated the effect of hyperbaric oxygen therapy (HBOT) on rat muscles during tibial distraction osteogenesis (DO) at normal and hyperdistraction rates. Animals in groups 1 and 2 were distracted by 0.5 mm/day and those in groups 3 and 4 by 1 mm/day. Groups 2 and 4 received HBOT during distraction. Group 5 served as control. Superoxide dismutase (SOD; U/g protein), malondialdehyde (nmol/g protein), glutathione (mmol/g protein), and protein levels (g/dl) were determined. SOD was significantly higher in group 2 (4.59 ± 0.97) than in controls (2.19 ± 0.7) (P = 0.0001), and lower in group 4 (3.74 ± 1.70) than in group 2 (P=0.011). Malondialdehyde was significantly higher in group 2 (0.72 ± 0.23) than in controls (0.38 ± 0.10) (P=0.005). Total protein levels were better preserved with HBOT in distracted muscles: group 2 (3.24 ± 0.37) vs. group 1 (1.88 ± 0.60), and group 4 (3.45 ± 0.70) vs. group 3 (2.03 ± 0.75) (both P=0.0001). Numbers of fibres were lower in group 1 (4.88 ± 0.59) than in group 2 (6.07 ± 0.86), and in group 3 (5.13 ± 0.36) than in group 4 (6.14 ± 0.74) (both P=0.001). Numbers of nuclei were higher in group 1 (11.29 ± 2.47) than in group 2 (9.03 ± 1.53) (P=0.04), and in group 3 (12.43 ± 3.32) than in group 4 (9.08 ± 1.58) (P=0.001). Fibres and nuclei with HBOT were similar to those of controls. HBOT decreased the inflammatory cell infiltrate for group 1 (19.8 ± 8.54) vs. group 2 (4.2 ± 2.53) and group 3 (36.54 ± 11.29) vs. group 4 (21.5 ± 9.23) (both P=0.001). HBOT improves the adaptation of distracted muscle by increasing fibres and antioxidants while decreasing inflammation.

Use it or lose it: multiscale skeletal muscle adaptation to mechanical stimuli

Skeletal muscle undergoes continuous turnover to adapt to changes in its mechanical environment. Overload increases muscle mass, whereas underload decreases muscle mass. These changes are correlated with, and enabled by, structural alterations across the molecular, subcellular, cellular, tissue, and organ scales. Despite extensive research on muscle adaptation at the individual scales, the interaction of the underlying mechanisms across the scales remains poorly understood. Here, we present a thorough review and a broad classification of multiscale muscle adaptation in response to a variety of mechanical stimuli. From this classification, we suggest that a mathematical model for skeletal muscle adaptation should include the four major stimuli, overstretch, understretch, overload, and underload, and the five key players in skeletal muscle adaptation, myosin heavy chain isoform, serial sarcomere number, parallel sarcomere number, pennation angle, and extracellular matrix composition. Including this information in multiscale computational models of muscle will shape our understanding of the interacting mechanisms of skeletal muscle adaptation across the scales. Ultimately, this will allow us to rationalize the design of exercise and rehabilitation programs, and improve the long-term success of interventional treatment in musculoskeletal disease.

Angiogenesis and myogenesis in mouse tibialis anterior muscles during distraction osteogenesis: VEGF, its receptors, and myogenin genes expression

Angiogenesis and myogenesis occur in the surrounding skeletal muscles following distraction osteogenesis, but their molecular mechanisms remain unclear. The present study investigated morphological features of lengthened muscles and the time course change of vascular endothelial growth factor (VEGF), its receptors (VEGFR-1 and VEGFR-2) and myogenin gene expression profiles related to angiogenesis and myogenesis in tibialis anterior (TA) muscles with a mouse model of distraction osteogenesis, which involves 1 week of waiting period (latency phase), 2 weeks of intermittent distraction (distraction phase), and 5 weeks of remodeling period (consolidation phase). Macroscopic findings showed that lengthened TA muscles increased to approximately 42% longer and 10% heavier at the end of the process when compared to pre-surgery. During the distraction phase, VEGF and its receptors were induced in the vascular endothelial cells, myogenin-positive satellite cells and myocytes, and subsequently, capillary progression and myogenesis were increased. Real-time RT-PCR showed that Vegf, Vegfr-1, Vegfr-2, and myogenin genes expression was enhanced during the muscle lengthening. Vegf and Vegfr-1 were upregulated following the recession of angiogenesis at the consolidation phase. We conclude that upregulation of VEGF and its receptors by mechanical tension-stress could be involved in the process of angiogenesis and myogenesis in lengthened muscles.

A rabbit model for studying degeneration and regeneration properties of young striated muscle at different distraction rates

The present study evaluated the histological changes in the muscle tissue after limb lengthening in skeletally immature rabbits and assessed the effect of different lengthening rates on the regeneration and degeneration properties of striated muscle. Thirteen different lengthening protocols were applied on a total of 16 male domestic white rabbits divided into four groups. The histopathological changes were analysed by a semiquantitative method according to the scoring system of Lee et al. (1993). After evaluation of the five main degenerative parameters (muscle atrophy, internalisation of muscle nuclei, degeneration of the muscle fibre, perimysial and endomysial fibrosis, haematomas), it is evident that rabbits subjected to limb lengthening at a rate of 3.2 mm/day showed more degenerative changes than those limb-lengthened at 0.8 or 1.6 mm/day. Our study showed that the regenerative mechanisms were not endless. If the daily lengthening rate reached the 3.2 mm/day limit, the regenerating ability of the muscle decreased, and signs of degeneration increased significantly.

Serial histologic and immunohistochemical changes in anterior digastric myocytes in response to distraction osteogenesis

PURPOSE

To document histologic and immunohistochemical changes in the anterior digastric muscle during distraction osteogenesis (DO).

MATERIALS AND METHODS

Nineteen Yucatan minipigs with mixed dentition were used for these experiments. Group A (n = 16) underwent unilateral mandibular distraction at a rate of 1 mm/day (no latency) for 12 days. Animals were killed at mid-DO (n = 5), end-DO (n = 5), mid-fixation (n = 4), and end-fixation (n = 2). Group B (n = 2) underwent acute 12-mm advancement, and group C (n = 1) dissection and osteotomy. Animals from groups B and C were killed at the end-DO time point. Digastric muscles from treatment and contralateral sides of all animals were harvested and embedded in paraffin. Specimens were stained with hematoxylin/eosin or immunohistochemically for proliferating cell nuclear antigen (PCNA; total cell proliferation), paired Box-7 gene protein (Pax7; satellite cells), or myogenic differentiation 1 protein (MyoD; differentiating myoblasts). Descriptive and bivariate statistics were computed to compare groups (P ≤ .05 statistically significant).

RESULTS

All animals survived the operation and observation period; there were no device failures. Two animals (1 at mid-DO, 1 at mid-fixation) were eliminated from the study because of postoperative infection. There was minimal digastric inflammation, fibrosis, and muscle fiber size variability during active DO. Immunohistochemical analysis showed statistically significant increases in PCNA (cellular proliferation), Pax7 (satellite cells), and MyoD (differentiating myoblasts) positive nuclei in digastrics at mid-DO and end-DO.

CONCLUSIONS

Results of this study indicate that there are minimal pathologic changes but significant increases in PCNA, Pax7, and MyoD positive nuclei during active distraction. This supports the hypothesis that the digastric muscle response to DO consists of proliferation and hypertrophy.

The adaptive response of jaw muscles to varying functional demands

Jaw muscles are versatile entities that are able to adapt their anatomical characteristics, such as size, cross-sectional area, and fibre properties, to altered functional demands. The dynamic nature of muscle fibres allows them to change their phenotype to optimize the required contractile function while minimizing energy use. Changes in these anatomical parameters are associated with changes in neuromuscular activity as the pattern of muscle activation by the central nervous system plays an important role in the modulation of muscle properties. This review summarizes the adaptive response of jaw muscles to various stimuli or perturbations in the orofacial system and addresses general changes in muscles as they adapt, specific adaptive changes in jaw muscles under various physiologic and pathologic conditions, and their adaptive response to non-surgical and surgical therapeutic interventions. Although the jaw muscles are used concertedly in the masticatory system, their adaptive changes are not always uniform and vary with the nature, intensity, and duration of the stimulus. In general, stretch, increases neuromuscular activity, and resistance training result in hypertrophy, elicits increases in mitochondrial content and cross-sectional area of the fibres, and may change the fibre-type composition of the muscle towards a larger percentage of slow-type fibres. In contrast, changes in the opposite direction occur when neuromuscular activity is reduced, the muscle is immobilized in a shortened position, or paralysed. The broad range of stimuli that affect the properties of jaw muscles might help explain the large variability in the anatomical and physiological characteristics found among individuals, muscles, and muscle portions.

Muscle response to leg lengthening during distraction osteogenesis

Continuous lengthening of intact muscles during distraction osteogenesis leads to an increase of sarcomeres and enhances the regeneration of tendons and blood vessels. A high distraction rate leads to an excessive leg and muscle lengthening and might cause damages of muscle fibers with fibrosis, necrosis, and muscle weakness. Complications like muscle contractures or atrophy after postoperative immobilization emphazize the importance of muscles and their function in the clinical outcome. In an animal model of distraction osteogenesis, 18 sheep were operated with an external fixator followed by 4 days latency, 21 days distraction (1.25 mm per day) and 51 days consolidation. The anatomical location (gastrocnemius, peroneus tertius, and first flexor digitorum longus muscle), dimension and occurrence of muscular defects were characterized histologically. The callus formation and leg axis was monitored by weekly X-rays. Additionally, serum creatine kinase was analyzed during a distraction and consolidation period. Significant signs of muscle lesions in all three observed muscles can be found postoperatively, whereas normal callus formation and regular leg axis was observed radiologically. The peroneus tertius and first flexor digitorum longus muscles were found to have significantly more signs of fibrosis, inflammatory, and necrosis. Creatine kinase showed two peaks: 4 and 39 days postoperative as an indication of muscle damage and regeneration. The study implicates that muscle damages should be considered when a long-distance distraction osteogenesis is planned. The surgeon should consider these muscle responses and individually discuss a two-stage treatment or additional muscle tendon releases to minimize the risk of muscle damages.

The effects of botulinum toxin A on muscle histology during distraction osteogenesis

Distraction osteogenesis is a highly successful method of bone formation, yet muscle fibrosis and contractures can result in significant morbidity. In the current study, we investigate the efficacy of botulinum toxin A in preventing fibrosis and potentially increasing muscle development in distracted muscles. Fifteen New Zealand White rabbits underwent tibial distraction at 1.5 mm/day until a 20% gain was achieved. Treatment groups were divided by drug (saline or botulinum toxin) and target muscle (gastrocnemius or tibialis anterior). Two additional control animals received no treatment. Bromeodeoxyuridine was delivered continuously throughout the 8-week experiment, and following muscle harvest. Tissues were stained for BrdU, Pax-7, vimentin, and haematoxylin and eosin staining. Mitotic activity increased in all distracted animals; however, in the animals receiving botulinum toxin A injections into the gastrocnemius, the antagonist tibialis anterior suffered up to 9% less fibrosis than distraction alone (p = 0.024). Use of botulinum A toxin did not appear to promote or improve neogenesis of muscle fibers, nor did it decrease fibrosis in the injected muscles. It appears from this study, and a previously published study on the effects of this toxin on muscle function, that botulinum A toxin maybe of some benefit in decreasing morbidity in the antagonist muscle but not the muscle injected with the toxin.

Minimizing pin complications when using the rigid external distraction (RED) system for midface distraction

In this review we describe the advantages, complications, and preventive considerations encountered as a result of the use of a halo for distraction of a retrusive nasomaxillary complex. Distraction osteogenesis is a well accepted combined orthodontic-surgical technique used in the treatment of patients with hypoplastic craniofacial components. The rigid external distraction (RED) system is a useful external distraction device for the advancement of severe retrusive maxilla especially in cleft palate patients. However, the addition of this new technique to the surgeon's armamentarium is accompanied by new complications and risks. Review of the literature on complications of the use of halo revealed that most complications are pin related. Complications with the use of RED have mainly included the penetration of intracranial pins. Risk management and preventive considerations propose several procedures to minimize the side effects when using RED: preoperative skull computerized tomography, pediatric neurosurgical consultation, proper pin care during distraction, frequent monitoring of the patient's general condition, proper pin and torque design, and special attention to the removal process of the RED.

Morphological and Immunohistochemical Changes in Muscle Tissue in Association With Mandibular Distraction Osteogenesis

PURPOSE

Studies of changes in the surrounding soft tissue in association with distraction osteogenesis in the maxillofacial region, where various different kinds of tissue are tightly packed, are rare. In this study, we performed morphological and immunohistochemical investigations of muscle tissue during mandibular distraction osteogenesis.

MATERIALS AND METHODS

Japanese white rabbits were divided into 2 groups. In 1 group, the mandibular bone was distracted at a rate of 1 mm/day (DO group), whereas in the other group it was advanced by 10 mm all at once (O group). The cross-section of the anterior belly of the digastric muscle was examined using hematoxylin and eosin, periodic acid-Schiff (PAS), and proliferating cell nuclear antigen (PCNA) staining methods, as well as scanning electronmicroscopy (SEM).

RESULTS

In the DO group, the cross-section of muscle bundle revealed no remarkable changes. In PAS staining, type II fibers gradually disappeared with distraction, but eventually reappeared after a period of consolidation. In the SEM photographs, striation-like laminated structures were evident in the control and the 3-mm distraction, ambiguous in the 6-mm distraction, and not evident in the 10-mm distraction. In contrast, the structure appeared to be restored in the consolidation for 1 week thereafter. In the PCNA immunostaining, the numbers of PCNA-positive nuclei increased during periods of distraction, but subsequently tended to decrease gradually.

CONCLUSIONS

Although minute injuries were induced in muscle fibers in association with distraction osteogenesis, it is suggested that muscle fibers regenerate starting in the distraction period and thus can adapt to the environment.

Effects of botulinum toxin A on functional outcome during distraction osteogenesis

Distraction osteogenesis is useful for correcting limb length inequality, deformities, or short stature. Despite success with bone formation, soft tissue maladaptations including muscle and joint contracture may lead to undesirable results. Botulinum toxin A has been useful in treating spasticity in cerebral palsy, and has been used clinically in select cases to allay contracture in distraction osteogenesis. This study examines the toxin's efficacy in preventing distraction-induced loss of muscle strength and range of motion. The left tibias of 15 New Zealand White rabbits were distracted 1.5 mm/day until approximately a 20% gain was achieved. Each treatment group was divided into animals injected with saline or botulinum toxin in either the gastrocnemius or tibialis anterior muscles. A control group of two additional animals underwent no surgical procedure. Strength and range of motion were assessed prior to, and following, the experiment. At the study's end, animals were euthanized and muscles were harvested, when lengths and weights were recorded. All muscles injected with botulinum toxin showed decreased wet weight and persistent weakness upon completion of the study. Range of motion decreased in all distracted animals. When the gastrocnemius was injected, its strength was reduced but the tibialis anterior strength was preserved, and the limb achieved 22% greater dorsiflexion than saline controls (p = 0.016). When the tibialis anterior received the toxin, plantarflexion was increased by 23% (p = 0.049). Botulinum toxin injection prior to limb distraction increases the "post-lengthened" excursion of the injected muscle and this increased length may have a protective effect on its antagonist. In toxin-injected gastrocnemius muscles, the level of equinus contracture is reduced due to length gains in the Achilles tendon while the anterior tibialis maintains its ability to generate torque. Injection of botulinum toxin in the gastrocnemius may minimize equinus contracture and protect the anterior tibialis from damage during human tibial lengthening. Longer follow-up studies are needed to ensure that toxin-induced muscle weakness resolves with time.

Mandibular distraction osteogenesis in a skeletal Class II patient with obstructive sleep apnea

We report the orthodontic treatment of a 31-year-old man with severe skeletal Class II malocclusion and documented obstructive sleep apnea (OSA). He had a retrognathic profile with an overbite of 4 mm and an overjet of 14 mm. Mandibular distraction osteogenesis was performed to lengthen the small, retruded mandible by 18 mm and improve the symptoms of OSA. Orthodontic treatment after the mandibular distraction osteogenesis procedure lasted 3 years 1 month. An acceptable occlusion was obtained, and the patient's OSA was significantly alleviated. Although the patient was satisfied with the treatment, condylar resorption was observed. The relevance of condylar resorption with reference to a comprehensive evaluation of the treatment outcome is discussed.

Neuromuscular recovery after distraction osteogenesis at different frequencies in a rabbit model

The muscle and nerve responses to stresses applied during distraction osteogenesis have not been clearly defined. This study hypothesized that distraction resulting in 30% lengthening decreases muscle force generation of the lengthened muscle and increasing the frequency of distraction attenuates the decrease of force generation accompanying lengthening. This study investigated the effects of different distraction frequencies on neuromuscular recovery in a rabbit model. Animals were assigned into group 1 (low-frequency distraction) and group 2 (high-frequency distraction). Distraction was continued until a 30% increase in the original tibial length was achieved. After consolidation of the osteotomy, knee and ankle range of motion, muscle force generation, and neuromuscular junction parameters were evaluated. Lengthening of 30% resulted in significantly decreased range of motion compared with the control leg (P < 0.05). Lengthening of 30% also substantially decreased force generation of the peroneus longus muscle. However, force generation of the peroneus longus muscle in the high-frequency group was 70.5% +/- 6.5% of the control side, significantly higher than that in the low-frequency distraction group (49.7% +/- 4.8% of the control side, P < 0.05). There was no statistical difference between the 2 groups in neuromuscular junction morphology, although an abnormal shape of the postsynaptic neuromuscular junction was observed after distraction. The use of a high-frequency distraction technique during limb lengthening may result in a reduction in impairment of knee and ankle range of motion and improved muscle function compared with that observed with the use of low-frequency distraction. Repeated microtrauma to the soft tissues associated with high-frequency distraction may facilitate the regenerative capacity of the soft tissues and result in an improved outcome of muscle and nerve function.

The Effect of Rate of Distraction Osteogenesis on Structure and Function of Anterior Digastric Muscle Fibers

The authors hypothesized that distraction at a rate of 3 mm/day, compared with mandibular distraction at a rate of 1 mm/day, would produce a maladaptive response in adjacent muscles of mastication. The authors further hypothesized that the maladaptive response would manifest at the single fiber level by means of increased sarcomeric heterogeneity, decreased maximum force output, and increased susceptibility to stretch-induced injury. In an ovine model, distraction osteogenesis of the right hemimandible was performed at either 1 mm/day for 21 days (n = 2) or 3 mm/day for 7 days (n = 2) to achieve a total distraction distance of 21 mm. The left hemimandibles served as controls. After a consolidation period of 2 days, the anterior digastric muscles were harvested; in six randomly selected single fibers from each muscle, maximum calcium-activated force (Po) was measured at optimal sarcomere length. The amount of damage to the sarcomeres in each fiber was assessed microscopically. To test susceptibility to contraction-induced injury, each fiber was given an activated stretch of 20 percent. Compared with control fibers and fibers distracted at 1 mm/day, maximum tetanic force (Po) was significantly lower in fibers distracted at 3 mm/day. Compared with control fibers, specific Po (Po/cross-sectional area) was lower in fibers distracted at 3 mm/day. The number of sarcomeres appearing damaged in fibers distracted at 3 mm/day was significantly higher than in control fibers or in fibers distracted at 1 mm/day. A greater deficit in Po was observed after a single activated stretch in fibers distracted at 3 mm/day than in control fibers or in fibers distracted at 1 mm/day. The authors conclude that distraction of the anterior digastric muscle in sheep at 3 mm/day produces a maladaptive response in the muscle fibers but a rate of 1 mm/day is tolerated by the muscle fibers. These data are consistent with the hypothesis that distraction of skeletal muscle at high rates results in increased heterogeneity of sarcomere lengths and that this increase in heterogeneity is the most likely potential mechanism resulting in whole muscle force deficits and in increased susceptibility to stretch-induced injury in distracted muscles.

The role of bone morphogenetic proteins BMP-2 and BMP-4 and their related postreceptor signaling system (Smads) in distraction osteogenesis of the mandible

Distraction osteogenesis has become a widely used clinical approach in the treatment of craniofacial and orthopedic disorders. The exact biological mechanism of bone formation remains illusive, however. The aim of this study was to evaluate the role of bone morphogenetic protein-2, bone morphogenetic protein-4, and transforming growth factor-beta superfamily-related postreceptor signaling glycoproteins Smads 1 through 5 in distraction osteogenesis. Twelve sheep randomly divided into two groups were distracted to 24 mm at 1 or 4 mm/d using a submandibular osteotomy and an external distractor. After a 5-week fixation period, the mandibles were harvested. Employing immunohistochemical techniques, the sections were investigated for the previous antigens. Osteoblasts and periosteal lining cells were strongly positive. The matrix did not stain for the antigens investigated. Osteocytes demonstrated weak staining for the antigens. No significant difference between the groups was noted. In fracture healing, bone morphogenetic proteins 2 and 4 have been localized to the cambial layer of the periosteum, where healing occurs by intramembranous ossification. Their diffuse staining of the osteoblasts in the distracted region supports a similar role in distraction osteogenesis, where bone formation is predominantly through intramembranous ossification. Furthermore, bone morphogenetic proteins 2 and 4 have been demonstrated to promote mesenchymal cell conversion to osteoblasts. This is similar to the process observed in distraction osteogenesis. The presence of related Smads confirms postreceptor activity of these bone morphogenetic proteins in the process of distraction osteogenesis. This study supports induction of bone morphogenetic proteins 2 and 4, their related postreceptor signaling system (Smads), and intramembranous bone formation associated with mechanical strain in distraction osteogenesis.

Alterations of morphology and microdensity in the condyle after mandibular osteodistraction in the rat

PURPOSE

In this study, we examined the effects of mandibular distraction osteogenesis on the morphology and the microdensity of the rat condyle.

MATERIALS AND METHODS

One hundred twenty-nine rats were allocated to 4 experimental groups (n = 32 or 33). Each received unilateral mandibular ramus osteotomy and distraction device placement. After a 3-day latency, these were distracted once a day for 5 days. The slow distraction group was distracted a total of 1 mm (0.2 mm/d); the moderate group, 2 mm (0.4 mm/d); the rapid group, 3 mm (0.6 mm/d); and the sham group, no distraction (0.0 mm/d). Eight to 9 rats in each group were sacrificed at each of 4 time points after device placement (6, 10, 24, and 38 days). Baseline data were obtained 3 days after osteotomy and device placement without distraction from an additional 10 rats. Radiographs of the hemimandibles were scanned and measured to evaluate changes in condylar size (height, width, and area), angulation, and bone microdensity converted to equivalent bone thickness using a stepwedge. The wet weights of masseter muscle were measured at the time of harvest.

RESULTS

1) Muscle weight gains over time were significantly lower in the treated than the untreated sides throughout the consolidation period (P <.001). 2) Condylar size and angulation on the untreated side increased postoperatively, whereas there was a significant reduction of these parameters (P <.01 to.001) on the treated side at 24 and/or 38 days. 3) Condylar microdensity significantly increased on the untreated side at 24 and 38 days (P <.05 to.01) but not on the treated side. 4) Faster or larger distraction caused more severe size reduction and more upright condylar angulation, prevented an increase in bone microdensity on the treated side, especially during the consolidation periods (P <.05 to.01), and retarded increase in muscle weight, whereas a slower distraction rate showed few negative, and even some positive effects. 5) Correlations in size, angulation, and microdensity between right and left condyles became less significant over time. 6) There were positive correlations between muscle weight and condylar size, angulation, and microdensity.

CONCLUSION

An increased rate of mandibular distraction has significant negative effects on condylar morphology and microdensity.

Unilateral advancement of the maxillary minor segment by distraction osteogenesis in patients with repaired unilateral cleft lip and palate: report of two cases

OBJECTIVE

Collapse of the maxillary minor segment with lateral crossbite is a common feature in patients with repaired unilateral cleft lip/palate because of maxillary alveolar bony defect and palatal scar tissue. Distraction osteogenesis (DOG) is an effective technique of lengthening and augmentation for bone and gingiva. This case report describes the effects of unilateral advancement of the maxillary minor segment by DOG in two patients with the repaired unilateral cleft lip/palate.

Histomorphometric evaluation of short-term changes in masseter muscle after lengthening the rabbit mandible by distraction osteogenesis

PURPOSE

Structural changes in muscles may affect the process during and after distraction osteogenesis (DO) of the mandible. However, the response of the masticatory muscles is still not well defined after gradual lengthening of the mandible. In this experimental study, short-term structural changes in masseter muscles of the rabbits are evaluated after mandibular DO.

MATERIALS AND METHODS

Left mandibles of 10 New Zealand rabbits were lengthened by DO for 7 days in the rate of 1 mm/day. Mandibles of all animals were removed at the end of the consolidation period. Muscle biopsy samples of distracted and contralateral sides were histopathologically investigated, and histomorphometric results were statistically analyzed.

RESULTS

Atrophy, hypertrophy, regeneration, and concomitant mild interstitial edema and fibrosis were found more evident in experimental side biopsy samples 30 days after distraction. Histomorphometric analysis revealed a statistically significant decrease in the mean regions of masseter muscle fibers of the distracted sides compared with the control sides (P <.05).

CONCLUSION

This experimental study showed that the structure of masseter muscle is influenced during and shortly after mandibular distraction osteogenesis. Atrophic changes of the ipsilateral masseter muscles may be regarded as regenerative response that occurs during and shortly after distraction period.

The adaptation of soleus and edl in a rat model of distraction osteogenesis: IGF-1 and fibrosis

The distraction rate of 0.5 mm/day produces good osteogenesis in small rodents; however, the effects of this distraction rate on muscle are not well documented. We evaluated the soleus and the extensor digitorum longus (EDL) after two weeks of lengthening distraction osteogenesis (DO) at 0.5 mm/day in skeletally mature rats. We found a modest but significant local increase of insulin like growth factor-1 (IGF-1) in the EDL, however, muscle growth indicated by developmental forms of myosin heavy chain (MHC) was not detected by mRNA (RT-PCR). To the contrary. the data suggested a decrease in cross-sectional area of the muscle fibers as well as a decrease in mRNA for slow MHC. Immunolabeling of fibronectin in cryosections of the EDL indicated fibrosis of the perimuscular connective tissue while assessment of the passive joint motion did not suggest a lack of excursion on the part of the dorsal flexors. While the literature suggests that IGF-1 facilitates muscle growth especially in young animals, excess of IGF-1 in muscle from adults may exacerbate DO-induced fibrosis.

Effects of distraction on muscle length: mechanisms involved in sarcomerogenesis

Although a great deal of interest has been given to understanding the mechanisms involved in regulating the radial growth that occurs because of resistance training, much less has been given to studying the longitudinal growth of skeletal muscle that occurs because of passive stretch. The current authors provide a brief overview of key issues relevant to the longitudinal growth of skeletal muscle that occurs during distraction osteogenesis. Specifically, five key issues are addressed: (1) the pattern of sarcomerogenesis during distraction; (2) sarcomerogenesis and altered expression of sarcomeric and nonsarcomeric genes; (3) the satellite cell hypothesis; (4) mitogenic factors; and (5) new approaches for studying the longitudinal growth of skeletal muscle. A discussion is provided that revolves around the concept of a negative feedback loop. One of the most interesting issues to be resolved in muscle biology is the role of satellite cells in regulating the growth of skeletal muscle. Currently, it is not known whether satellite cell activation is a prerequisite for the longitudinal growth of skeletal muscle. Gene chip analyses provide a paradoxical view, showing that distraction osteogenesis results in the upregulation of a gene, GADD45, involved with growth arrest and deoxyribonucleic acid destruction.

Lengthening of muscle during distraction osteogenesis

Chronic lengthening of immobilized, neurally intact muscle leads to the addition of sarcomeres in series. Confirmation of a similar adaptation during distraction osteogenesis is crucial for providing a rationale for a successful outcome of the intervention. When distraction osteogenesis (at < or = 1.4 mm/day) is done in skeletally immature animals, muscle adapts by creating a longer and functionally intact muscle. This is achieved through muscle growth, the proliferation of myogenic cells ultimately leading to serial addition of sarcomeres. When distraction osteogenesis is done in skeletally mature animals, however, the same distraction regimen leads to a lengthened muscle that has significant fibrosis and weakness, the latter possibly a result of partial denervation. Despite a modest but significant elevation of local insulinlike growth factor-1 in the lengthened muscles from adult animals, muscle growth is not adequate and leads to a loss of function. In adult animals, the distraction osteogenesis-induced increase in insulinlike growth factor-1 is insufficient to facilitate muscle growth during lengthening. Muscle can be targeted for future therapeutic use of insulinlike growth factor-1; however, such a therapy also may lead to increased fibrosis.

The effect of the amount of limb lengthening on skeletal muscle

The adaptation of tibialis anterior muscles after 20% and 30% gradual limb lengthening was evaluated. Eight skeletally mature neutered male goats had 20% (n = 4) or 30% (n = 4) tibial distraction at a rate of 0.25 mm three times per day. Muscles from lengthened and contralateral control limbs were harvested on completion of distraction. Fiber length and sarcomere length were measured followed by calculation of sarcomere number and muscle fiber-to-bone lengthening ratio. Fiber length and sarcomere number after 20% and 30% limb lengthening were significantly greater in the distracted muscles, whereas no difference in sarcomere length was detected. The difference in muscle fiber length and sarcomere number between distracted and control limbs was greater in the 30% than in the 20% group. The disproportion between the amounts of muscle fiber and bone length increase was similar after 20% and 30% lengthening. The results show that muscular adaptation continues during 20% to 30% limb lengthening by increasing fiber length. It seems that this increase occurs through serial sarcomere addition rather than sarcomere length alteration. The higher rate of musclerelated clinical complications after limb lengthening beyond 20% does not seem to be related to a failure of muscle fiber contractile elements to adapt to increasing limb length.

Experimental tooth movement through mature and immature bone regenerates after distraction osteogenesis in dogs

The purpose of this study was to verify the influence of tooth movement on tooth roots and periodontal tissues when teeth were moved into mature, well-organized, and mineralized regenerate bone created after distraction osteogenesis compared with immature, fibrous, and less-mineralized bone. Six 15-month-old male beagles underwent 10 mm of bilateral mandibular distraction osteogenesis. After 2-week (group 1) and 12-week (group 2) consolidation periods, third premolars were moved distally into the regenerate bone with 100 g of orthodontic force for 12 weeks. Simultaneously, second premolars were also moved distally as controls. After completion of tooth movement, the experimental animals were killed, and their tissues were harvested for histological evaluation. When premolars in groups 1 and 2 were compared, group 1 showed higher rates of tooth movement until the eighth week of experimental tooth movement (P <.05). The amount of tooth movement was significantly greater in group 1 than in group 2 or in the control teeth (P <.05). In group 1, we observed considerable root resorption extending into the dentin, and the thickness of the dentin became approximately half that of the controls at the compression side adjacent to the distraction gap. This root resorption extended from the cementoenamel junction to the root apex. In group 2, root resorption on the compression side reached the dentin, but the root resorption was less than in group 1. These results indicated that heavy force and early orthodontic tooth movement are not recommended when teeth are moved through regenerated bone created by distraction osteogenesis, to avoid tipping and severe root resorption.

Muscle adaptation during distraction osteogenesis in skeletally immature and mature rabbits

The lack of adaptation of muscle is thought to be a major source of complications during distraction osteogenesis (DO). Although adaptation to DO varies with the regimen (lengthening rate >1 mm/day and increase in bone length >20%) muscle contractures associated with DO may be a function of age. We tested this idea by subjecting skeletally mature and skeletally immature rabbits to an aggressive regimen of DO (1.4 mm/day with a 20% increase in tibial length). By using immunofluorescence to assess the presence of neonatal myosin heavy chain in sections from the tibialis anterior, we observed that the generation of new muscle tissue in response to DO was vigorous in young animals (27% positive fibers), whereas it was more muted in adult animals (9.9% positive fibers). This adaptive response was associated with a pronounced proliferation of myoblasts in the young but not in the mature animals. Adult tibialis anterior subjected to DO showed a 50% loss in tetanic and twitch tension whereas those in young animals did not. This correlated with partial denervation of adult but not young muscle, as judged by morphological criteria. These experiments indicate that adaptation to DO depends not only on mechanical variables but also on skeletal maturity.

The extracellular matrix of muscle--implications for manipulation of the craniofacial musculature

Successful adaptation of craniofacial skeletal muscle is dependent upon the connective tissue component of the muscle. This is exemplified by procedures such as distraction histo/osteogenesis. The mechanisms underlying remodelling of intramuscular connective tissue are complex and multifactorial and involve extracellular matrix (ECM) molecules, receptors for the ECM (integrins) and enzymes that remodel the ECM (MMPs). This review discusses the current state of knowledge and clinical implications of connective tissue biology as applied to craniofacial skeletal muscle.

Proliferation of masseter myocytes after distraction osteogenesis of the porcine mandible

PURPOSE

Long-term success of distraction osteogenesis depends on the ability of the surrounding soft tissues to tolerate distraction forces and to adapt to the resulting increase in skeletal length and volume. The hypothesis tested in this study was that mandibular elongation by distraction induces myocyte proliferation.

MATERIALS AND METHODS

Unilateral mandibular angle osteotomies were performed in 16 Yucatan minipigs. The hemimandibles were lengthened using semiburied distraction devices (Synthes Maxillofacial, Paoli, PA) with 0-day latency, 1, 2, or 4 mm/d distraction rates, and a neutral fixation period of twice the gap size in days. In 2 additional animals, the dissection and osteotomies were performed, and distraction devices were placed without activation (sham control). At the end of neutral fixation, tissues were taken from masseter muscle overlying the osteotomy and the equivalent region on the unoperated side (contralateral control). Proliferation of myocytes was estimated using immunohistochemical localization with antibodies against proliferating cell nuclear antigen (PCNA).

RESULTS

Muscle overlying the distracted mandible showed 6-fold more PCNA-positive myocytes (16.8% +/- 11.3%) than the contralateral control side (2.8% +/- 1.1%, P < .0001). In the 2 sham-control animals, there was a low index of PCNA-positive myocytes on both the osteotomy (2%, 5%) and the contralateral sides (1%, 2%).

CONCLUSIONS

The results of this study suggest that distraction of the porcine mandible by the protocol described induces myocyte proliferation in the masseter muscle. A proliferative response may contribute to improved long-term stability of mandibular expansion by distraction osteogenesis.

Application of passive stretch and its implications for muscle fibers

To increase range of motion, physical therapists frequently use passive stretch as a means of gaining increased excursion around a joint. In addition to clinical studies showing effectiveness, thereby supporting evidence-based practice, the basic sciences can provide an explanation how a technique might work once a technique is known to be effective. The goal of this article is to review the potential cellular events that may occur when muscle fibers are stretched passively. A biomechanical example of passive stretch applied to the ankle is used to provide a means to discuss passive stretch at the cellular and molecular levels. The implications of passive stretch on muscle fibers and the related connective tissue are discussed with respect to tissue biomechanics. Emphasis is placed on structures that are potentially involved in the sensing and signal transduction of stretch, and the mechanisms that may result in myofibrillogenesis are explored.

The adaptation of perimuscular connective tissue during distraction osteogenesis

It is not known whether the decreased range of motion observed during distraction osteogenesis results from the lack of adaptation of muscle or from fibrosis in the perimysium. The adaptation of the perimysium in the tibialis anterior muscle in skeletally immature rabbits using two distraction regimens (0.7 and 1.4 mm/day with 15% lengthening) was characterized. The resulting data indicate that during distraction osteogenesis, the muscle adapts by reorganization of its connective tissue. At a lengthening rate of 1.4 mm/day, there is perimysial fibrosis without major cellular pathologic abnormalities in the muscle fibers. The increase in perimysial thickness is characterized by an increase of collagen Type I. In addition, collagen Type I is deposited around the endomysium. The increase in total collagen and its cross-linking are dependent on the lengthening rate. The faster lengthening rate also leads to a significant decreased passive plantar flexion. Supplemental growth of the tibia was not observed, and a lack of adaptation in the muscle (based on resting length) was not seen. Together, the data suggest that decreased range of motion during distraction osteogenesis may be a function of the adaptation of the perimysium rather than of the muscle fibers.