The Modified Fels and Abbreviated Modified Fels Knee Skeletal-Maturity Systems in the Prediction of Leg-Length Discrepancy.

BACKGROUND: The Modified Fels (mFels) and Abbreviated Modified Fels (abFels) knee systems have been recently developed as options for grading skeletal maturity without the need for a separate hand radiograph. We sought to determine the interobserver reliability of these systems and to compare their prediction accuracy with that of the Greulich and Pyle (G-P) atlas in a cohort managed with epiphysiodesis for leg-length discrepancy (LLD). METHODS: Three reviewers scored 20 knee radiographs using the mFels system, which includes 5 qualitative and 2 quantitative measures as well as a quantitative output. Short leg length (SL), long leg length (LL), and LLD prediction errors at maturity using the White-Menelaus (W-M) method and G-P, mFels, or abFels skeletal age were compared in a cohort of 60 patients managed with epiphysiodesis for LLD. RESULTS: Intraclass correlation coefficients for the 2 quantitative variables and the quantitative output of the mFels system using 20 knee radiographs ranged from 0.55 to 0.98, and kappa coefficients for the 5 qualitative variables ranged from 0.56 to 1, indicating a reliability range from moderate to excellent. In the epiphysiodesis cohort, G-P skeletal age was on average 0.25 year older than mFels and abFels skeletal ages, most notably in females. The majority of average prediction errors between G-P, mFels, and abFels were <0.5 cm, with the greatest error being for the SL prediction in females, which approached 1 cm. Skeletal-age estimates with the mFels and abFels systems were statistically comparable. CONCLUSIONS: The mFels skeletal-age system is a reproducible method of determining skeletal age. Prediction errors in mFels and abFels skeletal ages were clinically comparable with those in G-P skeletal ages in this epiphysiodesis cohort. Further work is warranted to optimize and validate the accuracy of mFels and abFels skeletal ages to predict LLD and the impact of epiphysiodesis, particularly in females. Both the mFels and abFels systems are promising means of estimating skeletal age, avoiding additional radiation and health-care expenditure. LEVEL OF EVIDENCE: Prognostic Level II . See Instructions for Authors for a complete description of levels of evidence.

Ipsilateral Healthy-segment Response to Leg Length Discrepancy.

BACKGROUND: Few studies address frequency or magnitude of healthy lower-extremity segment response to ipsilateral companion segment shortening. We sought to document and quantify this occurrence in a variety of pediatric etiologies. METHODS: We reviewed the medical record and radiographs of patients undergoing epiphysiodesis to manage leg length discrepancy. Inclusion criteria for this study were leg length discrepancy of a single lower-extremity segment by identifiable cause and adequate scanograms to allow accurate measurement of all 4 lower-extremity segments before any surgical treatment for the discrepancy. We recorded the etiology of shortening, age of onset of disorder, the length of the lower-extremity segments on scanograms, and age at the time of radiographs. We considered ipsilateral healthy-segment difference from the contralateral ≥ 0.5 cm. as clinically significant. RESULTS: Two hundred nine patients met inclusion criteria (126 boys, 83 girls). The average age was 12.5 years. 16/60 patients with avascular necrosis of the hip demonstrated ipsilateral tibial shortening averaging 1.2 cm whereas 6/60 demonstrated ipsilateral tibial overgrowth averaging 0.6 cm. 11/30 Legg-Perthés patients demonstrated ipsilateral tibial shortening averaging 0.7 cm; none had ipsilateral tibial overgrowth. 10/42 posteromedial bow patients had ipsilateral femoral shortening averaging 0.8 cm, whereas 6/42 had ipsilateral overgrowth averaging 0.8 cm. 13/48 with distal femoral physeal injury demonstrated ipsilateral tibial shortening averaging 1.2 cm, whereas 6/48 demonstrated ipsilateral tibial overgrowth averaging 0.8 cm. 8/29 tibial physeal injuries (proximal or distal) demonstrated ipsilateral femoral shortening averaging 1.1 cm. whereas 7/29 demonstrated ipsilateral femoral overgrowth averaging 0.7 cm. CONCLUSIONS: Although there are individual exceptions, the ipsilateral healthy segment does not grow appreciably more than the contralateral in patients with avascular necrosis of the hip, Legg-Perthés disease, or physeal trauma. The femur is not a significant component of shortening in patients with posteromedial bow. LEVEL OF EVIDENCE: Level III, retrospective review.

Clinical and Radiographic Assessment of Adult Function After Blount Disease in Childhood: An Exercise in Futility.

BACKGROUND: Few studies evaluate long-term function of patients with Blount disease. We sought to document patient radiographic and functional status 20 to 30 years after sugical treatment for infantile or adolescent Blount disease. METHODS: We reviewed the medical records and radiographs of patients operated at our institution for Blount disease between 1985 and 1995. Over a 4-year period, we recruited subjects for an IRB-approved call-back study. RESULTS: One hundred five patients were eligible for the call-back study. Thirty-one (30%) had a criminal record, 18 of sufficient gravity to preclude invitation to return. Of the remaining 87, 40 (46%) could not be contacted. Of 47 with valid contact information, 10 (21%) were reported as deceased (although this could not be objectively confirmed), 20 (43%) did not respond or failed to show for assessment, and 1 (2%) declined to participate. 16 subjects returned at average age 36, 22 to 31 years post-index surgery. Body mass index (BMI) averaged 45.8 (range 23.9 to 67.6). Physical Score correlated most strongly and inversely with BMI ( P <0.01). Satisfaction with life correlated strongly and inversely with mechanical axis deviation ( P =0.02) and radiographic osteoarthritis of the knee ( P =0.02), but not BMI. There also was no correlation between severity of radiographic osteoarthritis and mechanical axis deviation ( P =0.46) or BMI ( P =0.52). CONCLUSIONS: The small fraction of patients returning for evalutation minimize clinical conclusions that can be drawn from this study, despite an intensive 4-year effort to conduct it. Management of obesity and other socioeconomic characteristics are likely the most important aspects of treatment of patients with Blount disease. Our primary conclusion is that meaningful long-term functional studies of pediatric orthopaedic conditions will not be answered by retrospective call-back studies, and must be conducted within prospective registries and regular longitudinal follow-up. LEVEL OF EVIDENCE: III-Case-controlled study.

Comparison of "Human" and Artificial Intelligence Hand-and-Wrist Skeletal Age Estimation in an Epiphysiodesis Cohort.

BACKGROUND: We previously demonstrated that the White-Menelaus arithmetic formula combined with skeletal age as estimated with the Greulich and Pyle (GP) atlas was the most accurate method for predicting leg lengths and residual leg-length discrepancy (LLD) at maturity in a cohort of patients treated with epiphysiodesis. We sought to determine if an online artificial intelligence (AI)-based hand-and-wrist skeletal age system provided consistent readings and to evaluate how these readings influenced the prediction of the outcome of epiphysiodesis in this cohort. METHODS: JPEG images of perioperative hand radiographs for 76 subjects were independently submitted by 2 authors to an AI skeletal age web site (http://physis.16bit.ai/). We compared the accuracy of the predicted long-leg length (after epiphysiodesis), short-leg length, and residual LLD with use of the White-Menelaus formula and either human-estimated GP or AI-estimated skeletal age. RESULTS: The AI skeletal age readings had an intraclass correlation coefficient (ICC) of 0.99. AI-estimated skeletal age was generally greater than human-estimated GP skeletal age (average, 0.5 year greater in boys and 0.1 year greater in girls). Overall, the prediction accuracy was improved with AI readings; these differences reached significance for the short-leg and residual LLD prediction errors. Residual LLD was underestimated by ≥1.0 cm in 26 of 76 subjects when human-estimated GP skeletal age was used (range of underestimation, 1.0 to 3.2 cm), compared with only 10 of 76 subjects when AI skeletal age was used (range of underestimation, 1.1 cm to 2.2 cm) (p < 0.01). Residual LLD was overestimated by ≥1.0 cm in 3 of 76 subjects by both methods (range of overestimation, 1.0 to 1.3 cm for the human-estimated GP method and 1.0 to 1.6 cm for the AI method). CONCLUSIONS: The AI method of determining hand-and-wrist skeletal age was highly reproducible in this cohort and improved the accuracy of prediction of leg length and residual discrepancy when compared with traditional human interpretation of the GP atlas. This improvement could be explained by more accurate estimation of skeletal age via a machine-learning AI system calibrated with a large database. LEVEL OF EVIDENCE: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Lower-Extremity Segment-Length Prediction Accuracy of the Sanders Multiplier, Paley Multiplier, and White-Menelaus Formula.

BACKGROUND: Several methods are available to estimate leg lengths at maturity to facilitate the determination of timing of epiphysiodesis. We compared the Paley multiplier, Sanders multiplier, and White-Menelaus methods in an epiphysiodesis-aged cohort. We assessed intra- and interrater reliability for Sanders skeletal stages and Greulich and Pyle atlas skeletal age. METHODS: Actual growth was recorded in healthy, unoperated femoral and tibial segments from an epiphysiodesis database. The predicted and actual lengths were compared with use of the Paley multiplier and White-Menelaus methods, Greulich and Pyle skeletal age, and the Sanders multiplier using Sanders stages. Intra- and interrater reliability were assessed in a separate group of 76 skeletal age films. RESULTS: The cohort included 148 femora and 195 tibiae in 197 patients. Femoral length at maturity was slightly underestimated by the Sanders multiplier and staging, was overestimated by the Paley multiplier and skeletal age, and was most accurately predicted with use of the White-Menelaus formula and skeletal age. All methods overestimated tibial length at maturity. The whole-leg prediction accuracy of the Sanders multiplier and White-Menelaus formula were comparable and were more accurate than that of the Paley multiplier. For Sanders skeletal staging, the interrater reliability varied from 0.86 to 0.88 and the intrarater reliability varied from 0.87 to 0.96. For Greulich and Pyle skeletal age, the interrater reliability varied from 0.87 to 0.89 and the intrarater reliability varied from 0.91 to 0.95. CONCLUSIONS: Use of the Sanders multiplier and skeletal stages was more accurate than the Paley multiplier and skeletal age in this cohort. Use of the White-Menelaus formula and skeletal age was slightly more accurate in predicting femoral length and slightly less accurate in predicting tibial length compared with the Sanders multiplier. Intra- and interrater reliability were similar between Sanders skeletal stages and Greulich and Pyle atlas skeletal age. The White-Menelaus formula and skeletal age was the recommended method for predicting lower-extremity segment lengths at maturity and epiphysiodesis effect. Although easier to recall without referencing an atlas and not sex-specific, Sanders skeletal staging does not correspond directly to years of growth remaining, and thus cannot be used with the White-Menelaus formula. CLINICAL RELEVANCE: The Greulich and Pyle atlas to determine skeletal age and the White-Menelaus formula to determine growth remaining are reliable predictors of epiphysiodesis effect in the lower extremities.

Is Proximal Fibula Epiphysiodesis Necessary When Performing a Proximal Tibial Epiphysiodesis?

BACKGROUND: Controversy exists regarding the need for proximal fibular epiphysiodesis in conjunction with proximal tibial epiphysiodesis to prevent relative overgrowth of the fibula. The purpose of this study was to determine the incidence of relative fibular overgrowth in patients who had undergone proximal tibial epiphysiodesis with or without proximal fibular epiphysiodesis to manage leg-length discrepancy. METHODS: We identified patients who had undergone proximal tibial epiphysiodesis, with or without concomitant fibular epiphysiodesis, followed to skeletal maturity, and with adequate scanograms to measure tibial and fibular lengths. We assessed tibial and fibular lengths, ratios, and distances between the tibia and fibula proximally and distally preoperatively and at skeletal maturity, and obvious radiographic proximal migration of the fibular head. RESULTS: A total of 234 patients met inclusion criteria, including 112 girls and 122 boys. In total, 179 patients had undergone concomitant fibular epiphysiodesis, and 55 had not. The fibular epiphysiodesis group was significantly younger preoperatively than the nonfibular epiphysiodesis group (average: 12.3 vs. 13.6 y), which accounted for most of the preoperative differences noted between the groups. Within the subset of younger patients (≥2 y of growth remaining at the time of epiphysiodesis), there were statistically significant differences between those with or without fibular epiphysiodesis at skeletal maturity in the proximal tibial-fibular distance (P<0.01) and the tibia:fibula ratio (0.96±0.02 vs. 0.98±0.02; P<0.02), but not in the distal tibial-fibular distance (P=0.46). Obvious fibular head proximal migration was noted in 10 patients, including 5/179 with concomitant proximal fibular epiphysiodesis, and 5/55 without (P<0.01). No patient was recorded as symptomatic with radiographic overgrowth, and no peroneal nerve injury occurred in any patient in this cohort. CONCLUSIONS: On the basis of this study, concomitant proximal fibular epiphysiodesis does not appear to be necessary in patients with 2 years or less of growth remaining, nor does it unequivocally prevent fibular head overgrowth. While the tibia:fibula ratio was quite consistent in general, there were individuals with relative fibular head prominence for whom fibular epiphysiodesis may be appropriate, particularly in relatively immature patients. LEVEL OF EVIDENCE: Level III-retrospective comparative study.

Comparison of Anderson-Green Growth-Remaining Graphs and White-Menelaus Predictions of Growth Remaining in the Distal Femoral and Proximal Tibial Physes.

BACKGROUND: Epiphysiodesis of the distal femoral and/or proximal tibial physes is commonly performed in children and adolescents to treat lower-limb length discrepancy. It is important to determine the amount of growth remaining in the physes of these patients. Two main methods are used to make this determination: the White-Menelaus arithmetic method, which assumes a consistent amount of growth per year until skeletal maturity, and the Anderson-Green growth-remaining charts, which diagram average growth per year ± 2 standard deviations. The purpose of the present study was to evaluate these apparently disparate predictions of growth remaining in a cohort of patients managed with epiphysiodesis. METHODS: From a database of patients managed with epiphysiodesis for lower-limb length discrepancy, we identified all healthy, unoperated leg segments with follow-up to skeletal maturity. We compared the amount of observed growth with the growth predicted by the White-Menelaus method and the Anderson-Green growth-remaining graphs, for both skeletal and chronological ages. RESULTS: A total of 441 healthy segments (201 femora and 240 tibiae) in 221 patients (105 boys and 116 girls) with an age range of 9.0 to 16.5 years at the time of epiphysiodesis were analyzed. We observed no relationship between the length percentile of the leg segments at the time of epiphysiodesis and the amount of growth that actually occurred. Growth in the distal femoral and proximal tibial physes was relatively constant (9 and 6 mm per year of skeletal growth, respectively). Skeletal age was a better predictor of growth remaining than chronological age for both methods of estimation. Although the distribution of growth remaining in the present study resembled the Anderson-Green growth-remaining charts, that distribution was not associated with the initial length of leg segments. CONCLUSIONS: The White-Menelaus method predicted the amount of growth remaining more accurately than the Anderson-Green growth-remaining charts. The distribution of growth remaining observed in the present study resembled that of the Anderson-Green growth-remaining graphs but was not predicted by the initial bone segment length. CLINICAL RELEVANCE: The present study found no positive association between the length percentile of leg segments and the amount of growth that occurred in an epiphysiodesis-age group. This finding can explain the comparative accuracy of the White-Menelaus method and the possible source of prediction errors in the straight-line graph and multiplier methods.

Timing of Epiphysiodesis to Correct Leg-Length Discrepancy: A Comparison of Prediction Methods.

BACKGROUND: The purpose of this study was to compare the accuracy of different methods used to predict ultimate leg lengths and residual leg-length discrepancy in a group of patients treated with epiphysiodesis at our institution. METHODS: Seventy-seven patients with adequate preoperative radiographs, no postoperative complications, and follow-up to skeletal maturity composed the study group. We compared the predicted lengths of both legs and residual leg-length discrepancy at maturity with actual outcomes using the White-Menelaus, Anderson-Green, Moseley, and multiplier methods. RESULTS: Skeletal age varied >1 year from chronological age in 61 (26%) of 231 observations, including 19 patients (25%) whose average skeletal age from 3 determinations differed by >1 year from chronological age. The prediction accuracy of each method was improved by using skeletal, rather than chronological, age. Error in prediction of the length of the short leg varied from a mean (and standard deviation) of 1.8 ± 1.2 cm for the straight-line graph to 2.5 ± 2.0 cm for the multiplier method. Prediction error for the long leg (after epiphysiodesis) varied from a mean of 1.2 ± 1.1 cm for the straight-line graph to 1.7 ± 1.5 cm for the multiplier method. Leg-length-discrepancy prediction error ranged from a mean of 0.7 ± 0.6 cm for the White-Menelaus method incorporating a growth inhibition factor to 1.1 ± 0.9 cm for the multiplier method. The multiplier method was the least accurate of all. All differences were significant. CONCLUSIONS: Determination of skeletal age is clinically relevant in patients being considered for epiphysiodesis to manage leg-length inequality. The multiplier method was the least accurate of the prediction methods in this patient population, which may have implications in calculating the appropriate timing of epiphysiodesis.

Complications Associated With Epiphysiodesis for Management of Leg Length Discrepancy.

BACKGROUND: Epiphysiodesis is well-established surgical treatment for the management of leg length discrepancy (LLD) in children; however, a variety of complications may occur. This study evaluates the nature, rate, and potential risk factors of complications associated with epiphysiodesis in a large patient population treated in 1 institution. METHODS: We evaluated the medical and radiographic records of 863 children who had lower extremity epiphysiodesis to manage LLD between 1980 and 2011. RESULTS: Sixty patients (7.0%) incurred complications of some type. Twenty-three patients (2.7%) had perioperative complications unrelated to physeal growth; all resolved without surgical treatment. These included 2 patients with transient intraoperative complications (laryngospasm and allergic rash), 7 with transient neurological symptoms (5 cutaneous nerve dysesthesia or numbness and 2 peroneal nerve neuropathies), and 14 with postoperative knee stiffness which resolved with therapy. Thirty-seven patients (4.3%) developed physeal growth-related complications, including 6 patients who developed overcorrection of leg length inequality (3 had contralateral epiphysiodesis to prevent overcorrection, while 3 declined), and 31 patients who developed angular deformity and/or continued growth of the physis. Of these 31 patients, 15 had reexploration of the epiphysiodesis site, 6 underwent corrective osteotomy, while in 10, no treatment was undertaken. Compared with patients who did not develop angular deformity, these 31 patients had significantly greater LLD (5.6 vs. 3.7 cm, respectively, P<0.01), were younger (10.7 vs. 11.7 in girls; 12.4 vs. 13.5 in boys; P<0.01), and were more likely to have a congenital etiology for their LLD (P<0.01). CONCLUSIONS: Epiphysiodesis is a safe and effective procedure for the correction of leg length inequality. The rate of complications in this study was 7.0%, the most prevalent being the development of angular deformity. Congenital etiology, younger age, and larger limb length inequalities were risk factors for the development of angular deformity. Both surgeons and families should be aware of nature and rate of complications associated with epiphysiodesis. LEVEL OF EVIDENCE: Level IV.

Deformity correction in pediatric skeletal dysplasia: treatment challenges and solutions.

We reviewed our experience with limb-deformity correction in 54 children with skeletal dysplasias. Our goal was to outline common treatment strategies developed in our hospital to overcome the challenges associated with the management of these conditions. Utilization of computer-assisted preoperative planning, intraoperative monitoring of peripheral nerve function, individualized bone segment stabilization using a modular circular external fixation system, and a flexible distraction protocol improved the precision of angular deformity correction in our practice, simplified external fixator assembly, diminished postoperative frame modifications, enhanced the stability of fixation, and reduced the rate of complications.

Citric acid-derived in situ crosslinkable biodegradable polymers for cell delivery.

Herein, we report a first citric acid (CA)-derived in situ crosslinkable biodegradable polymer, poly(ethylene glycol) maleate citrate (PEGMC). The synthesis of PEGMC could be carried out via a one-pot polycondensation reaction without using organic solvents or catalysts. PEGMC could be in situ crosslinked into elastomeric PPEGMC hydrogels. The performance of hydrogels in terms of swelling, degradation, and mechanical properties were highly dependent on the molar ratio of monomers, crosslinker concentration, and crosslinking mechanism used in the synthesis process. Cyclic conditioning tests showed that PPEGMC hydrogels could be compressed up to 75% strain without permanent deformation and with negligible hysteresis. Water-soluble PEGMC demonstrated excellent cytocompatibilty in vitro. The degradation products of PPEGMC also showed minimal cytotoxicity in vitro. Animal studies in rats clearly demonstrated the excellent injectability of PEGMC and degradability of the in situ-formed PPEGMC. PPEGMC elicited minimal inflammation in the early stages post-injection and was completely degraded within 30 days in rats. In conclusion, the development of CA-derived injectable biodegradable PEGMC presents numerous opportunities for material innovation and offers excellent candidate materials for in situ tissue engineering and drug delivery applications.

The role of variable muscle adaptation to limb lengthening in the development of joint contractures: an experimental study in the goat.

Muscle stiffness and joint contractures are currently regarded as the most common complications of limb lengthening. To better understand the mechanisms of joint contractures, architectural changes of all involved muscles were analyzed in 9 goats after 20% tibial lengthening with standard distraction protocol.All 13 muscles of the goat's tibia were found to be organized into an anterior compartment with 2 longitudinal and 4 pennate muscles and a posterior compartment with 1 longitudinal and 6 pennate muscles. Longitudinal muscles showed better compliance to distraction than pinnate muscles. Although muscle-to-bone lengthening ratio ranged widely (0-1.2), most of the muscles and especially those located in the posterior compartment showed much less lengthening than the bone. Muscular portions of the muscles lengthened more substantially (average, 17%) than their associated tendons (average, 7%). Muscle fiber length changes varied greatly between muscles (range, 0%-88%). Normalization of muscle fiber length revealed considerable elongation of anterior muscles fibers (25%) that was associated with an addition of new sarcomeres in series. Fiber length increase of all posterior muscles but one occurred by stretching of existing sarcomeres, with little addition or even dissolution of sarcomeres in series. This correlated with muscle mass changes showing significant muscle atrophy in the posterior compartment and better mass preservation in the anterior compartment.The study revealed striking difference in response to limb lengthening between individual muscles and muscles from antagonistic compartments in particular. Poor sarcomerogenesis in the posterior muscles leading to their insufficient length increase seems to play major role in the development of joint contractures.

Changes in pennate muscle architecture after gradual tibial lengthening in goats.

The purpose of this investigation was to examine the changes in unipennate muscle architecture after distraction osteogenesis. Nine adult goats underwent 20% tibial lengthening in one of the hind limbs. Immediately after distraction, lengthened and contralateral (untreated) tibialis caudalis (TC) muscles were harvested. Lengths of the muscle belly, muscle fiber (FL), sarcomere (SL), tendon (TL), and superficial aponeurosis, as well as muscle mass, pennation angle (PA), and physiological cross-sectional area (PCSA), were compared between the treated and contralateral sides. Lengthened TC muscle demonstrated 20.8% increase in belly length, 4.39% increase in TL, and 36.7% increase in FL, while PA decreased by 37.2% (P = 008). Muscle length increase was mainly due to lengthening of muscle belly, which resulted both from FL increase and 15.3% length increase in the aponeurosis component of muscle belly, without significant effect of the PA decrease. The FL increase was due to SL increase, not to sarcomere neogenesis, while mass and PCSA did not change. We concluded that although muscle architecture can be adversely affected by distraction because of deficient sarcomere neogenesis, PCSA can remain unchanged, giving false impression of preserved function. Change in PA plays only minimal role in muscle adaptation to distraction.

Acute deformity correction of lower extremities under SSEP-monitoring control.

Intraoperative somatosensory evoked potential (SSEP) monitoring was performed in eight children who had undergone an acute deformity correction in the lower extremities using external fixation. Five patients showed stable evoked potentials during surgery and had no neurologic complications postoperatively. Three patients experienced evoked potential abnormalities. In one patient, 60 degrees external rotation of the foot produced significant SSEP changes. The reduction of rotation to 40 degrees resulted in tibial but not peroneal SSEP recovery. Peroneal nerve deficit was noted postoperatively. The second patient showed substantial SSEP attenuation after 45 degrees correction of distal tibial valgus. However, spontaneous recovery of the response occurred, which allowed maintenance of the achieved correction. In a third patient, significant SSEP changes occurred after 90 degrees external rotation and 10 mm medial translation of the distal femur. Total release of translation allowed 75 degrees external rotation without SSEP abnormalities. Neither of the latter two patients had peripheral nerve deficits postoperatively. Intraoperative SSEP monitoring thus helps to define a neurologically safe limit of acute deformity correction.

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.