Cerebral palsy (CP) is a non-progressive neurological disorder in which the afflicted individual loses a certain degree of control over his or her muscles at an early age. People with CP have a wide range of conditions and skeletal problems, such as crouched gait, tightness in muscles, abnormal bone development, low bone density, impaired vision, and malnutrition. In addition, osteoarthritis arises prematurely and more often in patients with CP than in other individuals. Boldingh et al.1 studied 140 patients with CP ranging from 16 to 84 years of age and reported that 59% of them had osteoarthritis. Another study reported osteoarthritis in 27% of participants between the age of 15 and 25 years. 2 Both studies reported a higher prevalence of osteoarthritis than normal for each age group. Although many anecdotal observations of the occurrence of osteoarthritis of the hip joint in patients with CP have been made from surgical exposure and radiological evidence, no study has specifically detailed this correlation or the reasons behind it.
The morphogenesis, remodeling, and degeneration of diarthroidial joints are directly under the control of the loading histories created by the musculoskeletal system during development and aging. The altered loading histories in individuals with cerebral palsy (CP) lead to aberrations in joint morphogenesis and an acceleration of joint degeneration. To understand this process in the hip, the normal ontogeny of the hip joint is reviewed with special attention to the mechano-biological factors associated with joint morphogenesis, endochondral ossification, and cartilage degeneration. A contrast is then made with the mechano-biological alterations observed with CP and the consequent influence on joint destruction. The features of the pathogenesis are: (1) altered muscular activity and restricted range of motion result in abnormal joint morphology, subluxation, and poor coverage of the femoral head; (2) joint incongruities created in early development cause local stress concentrations that can mechanically damage the articular cartilage; (3) the reduced magnitudes of muscular forces reduce the contact pressures at the joints, creating thinner cartilage and osteopenia; and (4) the thinner cartilage degenerates early, and subchondral bone collapse further contributes to the mechanical destruction of the remaining cartilage.
Normal bone and joint development
To evaluate the effects of CP on osteoarthritis, one must appreciate the normal development of bone and cartilage. Bone growth and maintenance is a dynamic process that is regulated by biological and mechanical factors. Skeletal development begins in utero and is directed by the differentiation of mesenchymal stem cells into different cell or tissue types, including the cartilage anlagen or rudiments that serve as templates for the skeleton that later forms. 3 Eventually, the cartilage rudiments undergo endochondral and periosteal ossification to create bones. This growth and ossification are under the direct control of local mechanical-loading conditions created by muscular activity, which influences local gene expression.3
Articular cartilage is the hyaline cartilage at the end of long bones that facilitates smooth articulation of joints. It is the remaining part of the cartilage rudiment that has not undergone ossification. The preservation of articular cartilage is regulated by the various stresses and strains acting on it. Hydrostatic pressure on the cartilage promotes extracellular matrix production (aggrecan and collagen II) and inhibits cell hypertrophy. These factors slow ossification and maintain articular cartilage. Therefore, joint areas experiencing high forces have the thickest articular cartilage (Fig. 1).
Pathogenesis of idiopathic osteoarthritis with aging
Osteoarthritis is the destruction of the articular cartilage found at the joints of bones such as the hip. Osteoarthritis is most often associated with aging, where over time, the quiescent subchondral growth front slowly drifts toward the joint surface, thereby thinning the cartilage (Fig. 2a).4 Eventually the thinning articular cartilage loses its mechanical integrity and begins to wear away. As a result, people who suffer from osteoarthritis experience pain and inflammation at the joints when they move. However, the normally slow subchondral endochondral ossification and the subsequent mechanical destruction with aging are not the only mechanisms that can cause osteoarthritis. The degradation of cartilage can be accelerated by several other factors, including physical damage to the superficial cartilage, blunt impact, and joint laxity, which accentuates mechanical destruction of the articular cartilage.3
The process of subchondral endochondral ossification with aging can be accelerated by the lack of mechanical joint pressure. Joint loading creates hydrostatic pressure in the cartilage, which is chondroprotective.5 With improper or reduced loading of the joints, some areas of cartilage experience low hydrostatic pressure, which promotes endochondral ossification (Fig. 1).5
Another mechanism of cartilage destruction is surface wear or fibrillation of the articular cartilage (Fig. 2b, c). The cartilage tissue can be mechanically worn away by forces that rip or tear away pieces of cartilage. This mechanical degradation can occur rapidly and is accelerated by shear stresses that are created by incongruity between the joints.
Effects of CP on joint development and aging
People with CP develop a wide range of skeletal deformities shortly after birth. The more severe deformities include acetabular malformations, hip subluxation and dislocation, elevation of the tibial tubercle, fragmentation of the patella, increased femoral anteversion in the proximal femur, tibial torsion, and differing rates of skeletal maturation.6–8 Most of these changes are gradual and are caused by a multitude of factors ranging from abnormal joint forces to muscle imbalance caused by muscular spasticity.
Individuals with more severe cases of CP tend to develop osteoarthritis at a higher rate than those with less severe disease.2 The level of motor disability varies, but people with quadriplegical and diplegical are known to be more susceptible to osteoarthritis. The main reason is that their compromised ambulation, and their reduced muscular activity, as well as their restricted range of motion, is unable to provide sufficient cyclic loads to different areas of the hip that are necessary to maintain the cartilage. This promotes the development of osteopenia and accelerates subchondral endochondral ossification, making the cartilage thinner.
Subluxation and dislocation of the femoral head is a major feature of CP that contributes to osteoarthritis. Lundy et al.6 and Gamble et al.9 found that dislocations were more likely to be found in non-ambulators who are neurologically immature. Physiologically, the adductor, internal rotator, and hip-flexor muscle groups overpower their antagonistic groups and change the functional axis of the femur to the lesser trochanter.8,9 These forces create an imbalance of the hip and direct the head superolaterally, gradually causing it to subluxate and eventually dislocate. Subluxation comes with different degrees of gross and microscopic changes for the hip, such as deformation to the femoral head and an irregular physis.
The degeneration of the articular cartilage around the femoral head in patients with CP can be observed radiographically as well as by direct visualization during surgeries and autopsies of individuals with dislocated hips. The subluxation of the femoral head and the consequent morphological changes of the hip provide the mechanisms for osteoarthritis.10 It causes deformation of the femoral head with the flattening of the medial side of the epiphysis, causing the femoral head to have a wedge shape (Fig. 3).6 Moreover, a stress concentration created by the acetabular labrum can appear in the femoral head superolaterally and create a deep notch in the cartilage (Fig. 4). 6 This joint incongruity is a region of high stress that leaves the other parts of the femoral head with a reduced load. Areas of reduced loading are susceptible to osteoarthritis by undergoing the final stages of subchondral endochondral ossification. This is usually found on the medial side of the femur. On the other hand, even though high hydrostatic pressure is known to be chondro-protective, focal shear stresses are created in the notch where the head articulates against the acetabular labrum.8 Shear stresses are particularly damaging to the articular cartilage, and this region experiences wear and fibrillation.
Another common compounding problem found in those with CP is diminished bone mineral density (BMD). The pathogenesis of low BMD is complex, but it is caused primarily by low joint forces.11,12 Osteopenia and low BMD contribute to subchondral bone collapse and deformation of the femoral head. This malformation increases joint incongruity, further contributing to the mechanical damage of the articular cartilage.6
The abnormal loading conditions in CP lead to abnormal joint morphology and cause a cascade of events that lead to early osteoarthritis. This osteoarthritic cascade has four main features: (1) Altered muscular activity and restricted range of motion result in abnormal joint morphology, subluxation, and poor coverage of the femoral head. (2) Joint incongruities created in early development cause local stress concentrations that can mechanically damage the articular cartilage. (3) The reduced magnitudes of muscular forces reduce the contact pressures at the joints, creating thinner cartilage and osteopenia. (4) The thinner cartilage degenerates early, and subchondral bone collapse further contributes to the mechanical destruction of the remaining cartilage.