The mechanisms of relief from persistent pain after temporomandibular joint (TMJ) surgery are not well studied. It was hypothesized that if persistent pain is relieved by TMJ surgery, up-regulated parts of the central nervous system will be desensitized and the neuroendocrine opioid release will decrease back to normal levels. Eleven female patients with a mean age of 47.4 ± 19.4 years and with TMJ pain due to chronic closed lock were examined before and 6–24 months after TMJ discectomy. The effects on plasma β-endorphin levels, pain intensity, and pain thresholds were analyzed. Plasma β-endorphin levels ( P = 0.032), pain at rest ( P = 0.003), and movement-evoked pain ( P = 0.008) were all significantly reduced at follow-up. The reduction in plasma β-endorphin levels correlated with a reduction in maximum pain intensity ( P = 0.024) and with a longer time after surgery ( P = 0.041). Seven out of eight patients who reported a substantial reduction in maximum pain intensity presented a decrease in β-endorphin levels in the plasma. In conclusion, this pilot study showed a significant reduction in plasma β-endorphin levels and pain intensity at 6–24 months after TMJ surgery; plasma β-endorphin levels were correlated with time after surgery. However, the results must be interpreted with caution since this was a single-centre observational study with a small sample size. If replicated in larger sample sets, the measurement of β-endorphin levels may be of prognostic value for the treatment outcome.
Chronic closed lock of the temporomandibular joint (TMJ) is a disorder characterized by limited jaw function, pain at rest, and movement-evoked pain. It is often associated with degenerative and inflammatory changes of the disc, such as in osteoarthritis. The aetiology is still unclear, but systemic diseases, hormonal factors, ageing, functional overloading, increased joint friction, trauma, and general joint hypermobility may contribute to the condition.
In well-designed longitudinal follow-up studies, 85% of the patients have become pain-free after surgery. It has been recommended that both pain intensity at rest and movement-evoked pain should be assessed in patients undergoing surgery. Movement-evoked pain is usually more severe than pain at rest and is caused by activation of high-threshold mechanoreceptors, while the common belief is that pain at rest is caused by peripheral and central sensitization. Pain in other body locations, including back pain or headache, can be a predictor of persistent postoperative pain and thus ought to be considered.
In a study of osteoarthritis in human knees, pain intensity correlated poorly with the level of arthritic changes in the subchondral bone and with the presence of active inflammation, which has raised the possibility that central pain mechanisms participate in the process. In animal studies, it has been shown that afferent inputs from joints can induce plasticity in spinal dorsal horn neurons, leading to larger receptor fields and central sensitization. Previous results have shown that central sensitization is counteracted by increased descending inhibition.
During acute pain, β-endorphin is released from the pituitary gland into the blood stream upon activation of adrenergic receptors caused by the release of corticotrophin-releasing factor (CRF) from the hypothalamus. The hypothalamus in turn receives direct input from the dorsal horn and plays a central role in descending inhibition. It was demonstrated in a previous study by the present authors that patients with limited jaw opening and movement-evoked pain from the TMJ have higher plasma β-endorphin levels and lower pressure pain thresholds (PPTs) than healthy subjects.
It was hypothesized that if persistent pain is relieved by discectomy, a desensitization of the up-regulated parts of the central nervous system (CNS) will occur and thus central sensitization and neuroendocrine opioid release should decrease. The aim of this study was therefore to investigate plasma β-endorphin levels and pain variables before and after surgery in patients with TMJ chronic closed lock. In addition, potential correlations between plasma β-endorphin levels and pain variables were analyzed.
Patients and methods
Patient inclusion criteria
Patients with painful TMJ chronic closed lock were included. Due to the high female-to-male ratio of chronic closed lock patients seen at the study clinic (7:1), only female patients were included; this is in accordance with the recommendations proposed by the Sex, Gender and Pain Special Interest Group of the International Association of the Study of Pain. To be included in the study sample, the patient had to be >18 years of age, diagnosed with unilateral painful chronic closed lock of the TMJ (i.e. fulfilling the clinical criteria of disc displacement without reduction with or without limited opening and arthralgia according to the Research Diagnostic Criteria for TMD (RDC/TMD) ), and be planned for surgical treatment with discectomy according to the surgical diagnostic criteria. Comorbidities, such as pain in other body parts, were not considered, but it was ensured clinically that the TMJ pain was not referred from other structures, e.g. the jaw muscles or neck. Patients were excluded from the study if they had generalized joint diseases, for example rheumatoid arthritis.
Study design and procedure
A prospective study design was used and the study was performed at a university hospital in Huddinge, Sweden. Each patient was examined clinically with emphasis on chronic closed lock of the disc in the TMJ. Before being offered surgical treatment, patients presenting with pain from the masticatory muscles were treated conservatively with muscle exercises and an occlusal appliance for at least 3 months.
A panoramic radiograph was taken to detect any gross degenerative changes in the condyle, and thereafter a computed tomography (CT) scan was ordered. The CT scan was also performed to verify the anatomy of the joint. Magnetic resonance imaging was performed in two cases to investigate the pathology in the soft tissue, including the disc, which is often seen in patients with widespread or radiating pain . Patients were examined before and 6–24 months after surgery. Two surgeons trained in the same unit and using the same inclusion criteria for surgery (surgical diagnostic criteria categories 3 and 4) approved the patients for surgery, and one of them performed all discectomy procedures.
The study followed the principles stated in the Declaration of Helsinki Ethical Principles for Medical Research Involving Human Subjects and all patients gave their informed consent. The regional ethics review board approved the study.