Muscle contracture and passive mechanics in cerebral palsy

Abstract

Skeletal muscle contractures represent the permanent shortening of a muscle-tendon unit, resulting in loss of elasticity and, in extreme cases, joint deformation. They may result from cerebral palsy, spinal cord injury, stroke, muscular dystrophy, and other neuromuscular disorders. Contractures are the prototypic and most severe clinical presentation of increased passive mechanical muscle force in humans, often requiring surgical correction. Intraoperative experiments demonstrate that high muscle passive force is associated with sarcomeres that are abnormally stretched, although otherwise normal, with fewer sarcomeres in series. Furthermore, changes in the amount and arrangement of collagen in the extracellular matrix also increase muscle stiffness. Structural light and electron microscopy studies demonstrate that large bundles of collagen, referred to as perimysial cables, may be responsible for this increased stiffness and are regulated by interaction of a number of cell types within the extracellular matrix. Loss of muscle satellite cells may be related to changes in both sarcomeres and extracellular matrix. Future studies are required to determine the underlying mechanism for changes in muscle satellite cells and their relationship (if any) to contracture. A more complete understanding of this mechanism may lead to effective nonsurgical treatments to relieve and even prevent muscle contractures.

Keywords: biomechanics; cerebral palsy; extracellular matrix; sarcomere length; skeletal muscle mechanics.

Fig. 1.

Intraoperative measurement of dramatic sarcomere length increases in muscle contractures. A: intraoperative view of the anterior elbow of a person with an elbow contracture secondary to brain damage from prenatal cytomegalovirus infection. This image shows the gap created by surgical release (transection of the anterior fascia of the brachialis muscle and intramuscular tendon bundles and the biceps brachii distal tendon) before stair step lengthening. These releases allow passive elbow extension and, thus, basic activity of daily living functions such as self-care, feeding, and wheelchair driving. Intraoperative sarcomere length was measured by laser diffraction (17, 38) in upper extremity muscles from children with cerebral palsy (CP) or typically developing (TD) children or radial nerve injury patients: flexor carpi ulnaris muscle (B), soleus muscle (C), gracilis muscle (D), and semitendinosus muscle (E). (Data replotted from Refs. , , and .) Data represent means ± SE, n = 6–10 subjects/group. No error bars are provided for TD sarcomere lengths in D or E since these were single sarcomere length estimates based on a mathematical model.

Fig. 2.

Passive biomechanical properties of muscle specimens are altered in contractures because of cerebral palsy (CP) compared with typically developing (TD) muscle. A: bundle modulus measured in vitro from intraoperative muscle biopsies in children with CP is typically stiffer than that of bundles from TD children. B: titin molecular mass measured using specialized electrophoresis (70) from muscle biopsies reveals no meaningful differences between CP and TD muscles (note expanded vertical scale). C: in the gracilis muscle, linear regression reveals a weak correlation between muscle fiber stiffness and titin molecular mass (r² = 0.3) that is not significant (P > 0.2). This was typical for all muscles tested, suggesting that, in contrast to rabbit muscle (58), titin mass is not a major determinant of fiber stiffness in humans. D: modulus of three different specimen types from the mouse extensor digitorum longus muscle clearly demonstrates that the skeletal muscle extracellular matrix, present in fiber bundles only, bears most of the tensile load (for experimental details, see Ref. 50). SOL, soleus muscle; GR, gracilis muscle; ST, semitendinosus muscle; GAST, gastrocnemius muscle. Modulus data replotted from Refs. and . Data represent means ± SE, n = 6–10/group.

Fig. 3.

Extracellular matrix proteins change in contractures because of cerebral palsy (CP) compared with typically developing (TD) muscles. Cross sections of muscle from a TD child (A) and child with CP (B) immunolabeled with antilaminin antibody and imaged at the same magnification with the same laser intensity show increased thickness of laminin stain in CP. Calibration bar = 100 µm. C: collagen content (%collagen mass/muscle mass) measured by hydroxyproline assay of four different intraoperative muscle biopsies (SOL, soleus muscle; GR, gracilis muscle; ST, semitendinosus muscle; GAST, gastrocnemius muscle) varies by muscle type. D: relationship between collagen concentration (measured as hydroxyproline content) and tangent stiffness for CP muscle. Note that, in spite of the generally higher collagen content in CP muscle compared with TD, there is a clear lack of correlation between collagen content and stiffness in this human ST muscle (r² = 0.01, P > 0.8). Similar results were obtained for GR, SOL, and GAS. Ratios of collagen isoforms in CP muscle (E) and TD muscle (F) indicate no difference between CP and TD conditions. Data replotted from Refs. and . Data represent means ± SE, n = 6–24/group.

Fig. 4.

Images of perimysial collagen cables. A: low-magnification scanning-electron micrograph of perimysial cables running along the side of stretched mouse muscle fibers. B: higher-magnification scanning-electron micrograph of boxed area in A; this close-up of perimysial cables reveals the collagen fibrillar structure. C: sequential confocal images of a perimysial cable being stretched and rotating during imaging. Specimen was labeled with an anti-type I collagen antibody. Sarcomere length is shown in microns; perimysial cable angle is shown in degrees. D: 3-dimensional reconstruction of serial block-face images taken from mouse skeletal muscle color coded for capillaries (pink), fibroblasts (blue), or perimysial collagen cables (yellow). Scale bar = 5 µm. E: high-magnification (×11,000) transmission electron micrograph with stereology grid overlaid. Each point (at crosshair) was classified as a single collagen fibril (green circle), collagen fibril in cable (pink circle), or interstitial space (black). Points lying on objects that were not part of the extracellular space were not classified (red) or included in quantification. Scale bar = 250 nm. F: volume fraction of collagen fibrils located within perimysial cables determined by stereological analysis of transmission electron micrographs from transgenic mice lacking the desmin gene, resulting in fibrosis. (Images taken from studies published in Refs. –.)

Fig. 5.

Muscle morphology after serial sarcomere addition as a result of casting in normal and satellite cell-depleted muscle (see text for details). Hematoxylin- and eosin-stained sections of muscle with depleted satellite cell number (tamoxifen treated) (A) and vehicle control (B) after immobilization and stretch show smaller fibers and hypercellular extracellular matrix after satellite cell depletion. C: muscle fiber area in cross sections from tamoxifen-treated mice, vehicle control-treated mice, and uncasted mice. D: %muscle tissue from histological cross sections in tamoxifen-treated mice, vehicle control-treated mice, and uncasted mice. Data represent means ± SE, n = 4–5/group. (Data replotted from Ref. 28.)