Clinical and radiological symptoms of dislocated hip joint in DDH and CP are almost identical. Even surgical treatment methods of both groups are almost the same. However, the clinical practice shows that pain sensations of dislocated hip joint are more frequently observed in children with CP, which are born with normally developed hip joint, while the loss of contact between articular surfaces is most often observed in these children at the age after 3-5 years [10, 16, 17].
Numerous studies, concerning the distribution of nociceptive endings in the anatomical structures of various joints and the attempts to analyse their role in pain induction, have already been described in the literature [18–21]. However, there has, so far, been no comparison of nociceptor distribution in hip joint structures of patients with hip joint dislocation in the course of cerebral palsy and developmental dysplasia of the hip or a proper analysis, explaining the effect of differences in this distribution on the occurrence of joint pain sensations.
In our studied groups, pain occurred in more than a half of the patients with cerebral palsy, while such sensations were not observed in the patients with DDH. In all the children with cerebral palsy and cartilage defects, positive immunohistochemical reactions were found for S-100 protein, substance P or for both studied markers in the joint capsule and/or the femoral head ligament. In comparison with patients with hip joint dislocation in the course of developmental dysplasia, a positive immunohistochemical reaction to S-100 protein more frequently concerned structures of the femoral head ligament in children with CP and cartilage defects on the femoral head. Positive immunohistochemical reactions to S-100 protein were also more frequent in the joint capsules of children with CP and cartilage defects and pain sensations than in children with DDH. It was also found that a positive immunohistochemical reaction for substance P concerned more often the structures of the femoral head ligament in children with CP and cartilage defects or pain sensations, when compared to children with DDH.
Saxler et al. observed a 2.5× higher level of immunopositive nerve fibres for substance P and CGRP in structures of osteoarthritis hip joint vs. a control group . Similarly in our studies, a significantly higher content of immunopositive fibres for S-100 protein was found in the femoral head ligament and in the articular capsule of patients with CP, when compared with preparations from the patients with DDH.
The relationship, observed between the occurrence of substance P and S-100 protein and pain intensity in the groups of children with cerebral palsy and with DDH , unequivocally confirmed that pain in the previous group was associated with an increased expression of both these factors. It was particularly distinctive in the femoral head ligament, while less in the joint capsule. In patients, suffering from pain, both markers were higher vs. the group with DDH, in which no pain was observed. Also femoral head cartilage defects in the group of patients with CP were associated with a higher expression of both markers, both in the joint capsule and in the femoral head ligament. These differences emphasise the specific role of inflammatory process, present in defective tissues. This hypothesis was confirmed by authors, suggesting that proteins of the S-100 family, released into the extracellular environment, could play the role of a pro-catabolic and pro-inflammatory factors which stimulate cell proliferation and joint cartilage degradation [22, 23].
The results of our studies have also indicated a new, important aspect, concerning possible predispositions to hip joint pain in children with CP, depending on the density of nociceptor layout. In 8, out of eleven (72.7%) cases of children with CP, suffering from hip joint pain, there was a simultaneous increase in the number of immunopositive fibres for substance P and S-100 protein in the femoral head ligament. Pain intensity seems then to be related to concomitant localisation of substance P and S-100 protein, especially in the femoral head ligament. In the group of children with DDH, the lack of pain sensations correlate well with only limited, colocalisation of substance P- and S-100 protein-immunopositive fibres (13.3%).
The aetiology of pain sensations in the group with CP does not provide any clear indication why pain, associated with hip joint dislocation, occurs in some patients only. In histopathological studies, free nerve endings with nociceptive function were identified in all the joint structures except the articular cartilage. Despite the lack of innervation and vascularisation, cartilage defects induce pain sensations of, so far, non defined aetiology. It is very well possible that an accumulation of microtraumas may lead to cartilage wear, as well as to osteoporotic fractures . Also a gradual loss of the articular cartilage may lead to pain sensation and reduced articular mobility, resulting from an exposure of nerve endings in the subchondral layer.
It cannot be excluded that an increased density of the nerve fibres indicating expression for substance P, may be a consequence of direct and indirect inflammatory processes that take place in degenerative and defective joint structures around the cartilage [25, 26].
Some authors have found immunopositive fibres for substance P in articular structures, such as the synovial membrane, the meniscus and ligands . The presence of substance P-containing nerve fibres in the femoral head ligament, may suggest their important, nociceptive role in the hip joint, supportive for the development of pathophysiological processes in the femoral head ligament and the joint capsule. Their intensity and scope demonstrate among others Witoński and Wagrowska-Danilewicz . They found that the number of P-positive nerve fibres was not constant and might undergo a certain increase during a 4-month period from the injury. Also in our groups of patients, the time period of irritating the femoral head ligament and joint capsule tissues was different, what resulted from the already mentioned changes in the hip joint in the course of developmental dysplasia, which have their beginning already in the foetal life.
The question still remains unanswered, whether there is any relationship between the occurrence of hip joint pain in children with CP and the density of nociceptors. The results of studies by Maslon et al. , as well as of studies by other authors , confirm that there is such a relationship, moreover indicating a specific relationship between the density of substance P-positive fibres and pain occurrence. In hip joint dislocation in the course of spasticity as well as in the presence of other aetiological factors, leading to pain induction by the hip joint, an increased density of nociceptive fibres is closely related with pain intensity. The whole set of complex activities, including the activation of nerve fibres, immunopositive for substance P and S-100 protein, initiates important cellular processes which, in turn, trigger a number of secondary, intracellular transmitter systems, especially protein kinases .
So huge clinical differences between CP and DDH groups with dislocated hip joint, expressed in the frequency of pain, seems to taking its source in tissue sensitization and inflammatory process or articular cartilage defects. At present, the only way to stop the adverse biological processes is early surgical treatment.