Margie Mathewson1, Samuel Ward1,2,3, Henry Chambers4, and Richard Lieber1,3 Departments of 1Bioengineering, 2Radiology, and 3Orthopaedic Surgery, University of California San Diego, CA

4Division of Orthopaedics, Rady Childrens Hospital, San Diego, CA

INTRODUCTION

Stretched Sarcomeres May Contribute to Contracture in Cerebral Palsy

METHODS

RESULTS

DISCUSSION •  Muscle fascicles in patients with CP have the same fascicle length as TD patients because their sarcomeres

are highly stretched. •  At their in vivo sarcomere lengths, both fibers and bundles from patients with CP are significantly stiffer

than those of TD patients. •  These data suggest that, along with connective tissue dysfunction, contractures in CP may be related to

highly stretched sarcomeres, which create high passive muscle stiffness and reduced joint range of motion. •  Information regarding these long sarcomere lengths and high in vivo passive stiffness may help to advise

clinical practices such as stretching regimes and surgical interventions. References: 1. Barrett, R. S., Lichtwark, G. A., (2010). Gross Muscle Morphology and Structure in Spastic Cerebral Palsy: A Systematic Review. Developmental Medicine and Child Neurology 52(9), 794-804. 2. Shortland, A. P., Harris, C. A., Gough, M., and Robinson, R. O., (2002). Architecture of the Medial Gastrocnemius in Children with Spastic Diplegia. Developmental Medicine and Child Neurology 44, 158-163. 3. Lieber, R. L., Yeh, Y. and Baskin, R. J. (1984). Sarcomere length determination using laser diffraction. Effect of beam and fiber diameter. Biophysical Journal 45, 1007-1016. 4. Ward, S. R., Eng, C. M., Smallwood, L. H., and Lieber, R. L. (2009). Are Current Measurements of Lower Extremity Muscle Architecture Accurate? Clinical Orthopaedics and Related Research 467(4), 1074-1082.

Children with cerebral palsy (CP) can develop muscle-shortening contractures, leading to the assumption that muscle fascicles in children with CP are shorter than in typically developing (TD) children. Ultrasound data, however, are conflicting. We believe this conflict occurs because muscle sarcomere length (Ls), which ultrasound cannot measure, has not been correlated with ultrasound data. Adaptations in architectural properties, specifically in vivo Ls, can have a significant impact on the passive mechanical environment of muscle. We therefore set out to quantify fascicle length, Ls, and serial sarcomere number in CP and TD muscles, as well as to characterize passive mechanical muscle environment.

** This work was supported by NIH grant AR057393 and an NSF Graduate Research Fellowship (MAM).

Patients • CP: n=20, age=12.1±5.3 years • TD: n=21, age=12.4±3.4 years Experimental Procedure: • Obtained soleus ultrasound images • Ankle angles recorded for CP patients, TD measurements taken at CP average

• Measured distance between fascial planes (d) and angle of muscle fascicles (Ɵ) as previously described2 •  Lf=d/sin(Ɵ)

Figure 2. There was no difference (p>0.6) in average soleus fascicle length between CP (3.6±1.2 cm) and TD (3.5±0.9 cm). Results are shown as mean±SD.

dθ

• Collected muscle biopsy at angle corresponding to clamps

• Measured sarcomere length using laser diffraction3

• TD values taken from literature4

and adjusted for ankle angle

• Calculated serial sarcomere number: Ns=Lf/Ls

1.

2.

3.

Figure 1. Possible fascicle scenarios: 1. Sarcomere lengths the same in CP and TD

2. Longer sarcomeres in patients with CP

3. Shorter sarcomeres in patients with CP (contracted muscles)

Figure 3. Sarcomere lengths in patients with CP (4.07±0.45 µm) were significantly longer (p<0.0001) than previously reported sarcomere lengths in TD patients4 (2.17±0.24 µm).

Figure 4. Serial sarcomere number was significantly smaller (p<0.0001) in patients with CP (9,190±3,810) compared to TD individuals (16,040±4,160)

• Passive mechanical properties tested by stretching muscle fibers and fiber bundles incrementally §  3 min relaxation between stretches

• Measured force and sarcomere length for each stretch

• Calculated stress and tangent stiffness with respect to sarcomere length

Force transducer

Motor arm

Muscle fiber

Figure 6. At measured in vivo sarcomere lengths for CP and TD, CP fibers (CP=21.0±15.1 kPa/µm vs TD=2.5±4.6 kPa/µm) and bundles (CP=24.9±16.5 kPa/µm vs TD=0.3±7.3 kPa/µm) are significantly stiffer (p<0.0001).

Figure 5B. After a hypothetical muscle-tendon unit lengthening of 2.5 cm in the average patient, sarcomere length decreases to 3.8 µm and maximal theoretical force production increases to 31% of maximum.

Figure 5A. When calculated using muscle architecture, at the in vivo sarcomere length, an average patient with CP has a predicted muscle force production of only 13% of maximum.

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