Efficacy of Combination of Meloxicam and Pregabalin for Pain in Knee Osteoarthritis

Knee osteoarthritis (OA) is a common and progressive joint disease. With an estimated incidence rate of 240 per 100000 person-years, it is a major public health problem in the United States and often results in early retirement and joint replacement.1

Chronic pain can be classified as inflammatory or neuropathic. Inflammatory pain, found in conditions such as rheumatoid arthritis, is the result of sustained stimulation of nociceptors. By contrast, neuropathic pain is the result of damage to or dysfunction of nerves. Principal causes of neuropathic pain (NP) include diabetes, lumbar or cervical radiculopathies, and spinal cord injury.2, 3 OA of the knee is generally considered to result in inflammatory pain.

Intra-articular injection of monoiodoacetate into the femorotibial joint space of rodents induces a pathology with temporal similarities to OA, and this model of OA is now used for investigating the pathogenesis of knee pain.4 In the early phase of the rat model pathology, the level of inflammatory neuropeptides in the sensory nerves innervating the knee increases, and this is believed to be a stage of inflammatory pain.5 By contrast, because the subchondral bone is densely innervated, subchondral bone pathology may result in neuropathy after destruction of the chondral structure in late phases of OA. In the late phase of the pathology in the rat model of OA, markers of nerve injury in sensory nerves innervating the knee are significantly increased compared with controls, and this is thought to be a NP stage.6 The origin of the pain would be clarified by assessing whether nerve damage or neuropathy underlies the lack of pharmacological response to celecoxib or diclofenac, but not to morphine as observed later in the temporal sequence of the pathology.7, 8 These observations suggest that the pain of knee OA is both inflammatory pain and NP.

Patients with NP are challenging to manage. Available randomized clinical trials typically evaluated chronic NP of moderate to severe intensity. The trials concluded that first-line treatments could include certain antidepressants (i.e., tricyclic antidepressants and dual reuptake inhibitors of both serotonin and norepinephrine), calcium channel α2-δ ligands (i.e., gabapentin and pregabalin), and topical lidocaine.9 Pregabalin was developed for management of NP; however, there have been no reports of the clinical use of pregabalin for OA pain in patients.

Our hypothesis is that there is an inflammatory and a NP component to knee OA pain. Thus, the purpose of current study was to evaluate the efficacy of pregabalin for knee OA pain in patients using a prospective randomized trial.

The ethics committee of our institution approved the protocol for the human procedures used in this study. In addition, the protocol and publication of the study were approved by our institutional review board and informed consent was obtained from each participant. Participants were selected from outpatients who attended our hospital for knee pain treatment. Eighty-nine patients were selected from 130 knee pain patients who matched the following criteria: presence of knee pain for more than one month, and observation of OA of the knee joint on examination of an anterior-posterior X-ray image. Exclusion criteria were a history of knee surgery, infection, or rheumatoid arthritis. The patients completed a self-administered questionnaire regarding sociodemographic factors (age and sex) and duration of knee symptoms.

No patients dropped out of the current study. Table 1 shows demographic characteristics for patients. Eighty-nine patients (63 female, 26 male) with a mean age of 70.0±8.0 (mean±SEM) years were admitted into the study. The average duration of symptoms was 35.5±8.0 (mean±SEM) months. The patients suffered from knee pain, originating from OA, for at least one month. Pain using VAS and WOMAC scores was not significantly different among the three groups (p>0.05).

Table 1
Demographic Characteristics

• Click for larger image

Table 2 shows the painDETECT score before medication, indicating likely NP (score ≥19), possible NP (score ≥13 to ≤18), and unlikely NP (score ≤12). Within this study population, 6 (6.7%) were classified as likely NP, 16 (17.8%) as possible NP, and 67 (75.3%) as unlikely NP. The NP scores were not significantly different among the three groups (p>0.05).

Table 2
PainDETECT Score before Medication

• Click for larger image

Table 3 shows evaluation of KL grade. There was no patient with KL0, and all patients were distributed from KL1 to KL4. Twenty-two (24.7%) patients were classified into KL1, 24 (27.0) into KL2, 31 (34.8%) into KL3, and 12 (13.5%) into KL4. The percentage and number of each KL grade were not significantly different among the three groups (p>0.05).

Table 3
X-Ray Evaluation

• Click for larger image

Table 4 shows pain during medication. Overall, medication improved the pain score in the three groups compared with before medication. There was significant pain relief in the meloxicam+pregabalin group as assessed by VAS score at 1, 2, and 4 weeks, and WOMAC score at 4 weeks, compared with the meloxicam or pregabalin only groups (p<0.05). No significant pain relief was seen in the meloxicam only group as assessed by VAS score during 4 weeks or WOMAC score at 4 weeks compared with the pregabalin only group (p>0.05).

Table 4
Pain during Medication

• Click for larger image

In the current study, we investigated the efficacy of pregabalin for OA pain in OA patients. A combination of meloxicam+ pregabalin was more effective than meloxicam or pregabalin alone for pain in OA patients. This finding suggests that OA pain is a combination of inflammatory and neuropathic pain components.

The origin of OA pain is widely considered to be inflammatory, but our study identified at least 6.7% of our OA patients as likely to have NP and 17.8% as possibly having NP. Hochman, et al.14 used a list of NP descriptors from five validated questionnaires to evaluate knee OA pain. To distinguish inflammatory pain from NP in people with other chronic pain conditions, five items were commonly identified. These investigators reported that 34% of patients with OA of the knee described their pain in a way suggestive of NP.14 Some patients with severe pain and related disability may have a NP component in the pain of their knee OA.

Other investigators have reported that joint pain arises mainly from free nerve endings that exist in the capsule or in the synovium.15-17 By contrast, yet others have reported innervation of the osteochondral junction in human knee OA samples and suggested a contribution of the subchondral area to the pain of OA knees.18, 19 We hypothesize that NP occurs in association with damage to nerves innervating subchondral bone because of its weight-bearing surface in late stage OA. Approximately 60-80% of OA patients achieve pain relief after local anesthetic treatment or surgical replacement of the affected joint, indicating that peripheral mechanisms drive the pain, although central mechanisms are also thought to play a role in some patients, and dysfunction of diffuse noxious inhibitory controls has been described as an alternative hypothesis.20-22 In an animal model of OA, the gradual proliferation of microglia related to neuropathic pain in the dorsal horn of the spinal cord demonstrated progressive nerve injury, which suggests the gradual initiation of a neuropathic pain state along with an inflammatory pain state in OA.23 These central mechanisms may overlap with those of other pain states including neuropathy.

Pregabalin was developed for management of NP; however, there has been no report of its use for pain in OA patients. Rahman, et al.24 reported that in an animal model of OA, pregabalin is useful for pain derived from knee OA, and they showed an altered physiology of deep dorsal horn wide dynamic range neurons in their model of knee OA, indicating a link between increased neuronal activity and hypersensitive behavioral responses in these animals.24 However, hypersensitivity was suppressed by antinociceptive actions of pregabalin, suggesting descending facilitation.24 In this regard, unraveling the cellular and molecular mechanisms underlying the OA pain state is of major clinical importance in the development of more clinically effective drugs such as pregabalin.

There are some limitations to the current study. First, we used only one drug dose in the three groups. In particular, we used 25 mg pregabalin for pain in patients in this study. Generally, 150 mg of pregabalin is recommended for the treatment of NP in patients with peripheral nerve injury and diabetic neuropathy. We believe that 150 mg is excessive for OA patients and were concerned about adverse events. Therefore, we selected 25 mg pregabalin for the current study. Second, we used 30 patients in each group. This number is not sufficient for a power analysis. Further study using different dosages and a larger number of patients is needed to clarify these points.

In conclusion, a combination of meloxicam plus pregabalin was effective for OA pain in OA patients. This finding, furthermore, suggests that OA pain is a combination of inflammatory and neuropathic pain components.