Basic aspects of musculoskeletal pain: from acute to chronic pain

Abstract

The transition from acute to chronic musculoskeletal pain is not well understood. To understand this transition, it is important to know how peripheral and central sensitization are manifested and how they can be assessed. A variety of human pain biomarkers have been developed to quantify localized and widespread musculoskeletal pain. In addition, human surrogate models may be used to induce sensitization in otherwise healthy volunteers. Pain can arise from different musculoskeletal structures (e.g. muscles, joints, ligaments, or tendons), and differentiating the origin of pain from those different structures is a challenge. Tissue specific pain biomarkers can be used to tease these different aspects. Chronic musculoskeletal pain patients in general show signs of local/central sensitization and spread of pain to degrees which correlate to pain intensity and duration. From a management perspective, it is therefore highly important to reduce pain intensity and try to minimize the duration of pain.

Keywords: Sensitization; experimental pain; hyperalgesia; joint; muscle.

Figure 1

Example of the pain distribution after injecting a small bolus of hypertonic saline into the muscle, tendon and tendon-bone junction of the tibialis anterior. Larger referred pain areas and higher pain intensities were often found after injecting the tendon-bone junction compared with tendon and muscle. Based on data from Gibson et al.

Figure 2

Mapping of the pressure pain sensitivity. Pressure pain thresholds are assessed on several sites around the knee in a non-symptomatic healthy subject and in three different OA pain patients. A novel pain sensitivity mapping approach illustrates the individual pattern of sensitivity changes in patients. Based on data from Arendt-Nielsen et al.

Figure 3

If a pain patient with an initial musculoskeletal pain problem is followed over years and if the problem is not resolved the pain starts to spread outside the origin of pain due to development of central sensitization. The pain will gradually spread as illustrated on this sketch and the spread can often not be explained by e.g. disease progression.

Figure 4

The pressure pain sensitivity can be assessed by the cuff-algometry method where a tourniquet is automatically is inflated and the subject simultaneously evaluates the pain intensity on an electronic Visual Analogue Scale (VAS). The pain threshold is defined as the first time the VAS score exceeds 0 and the Pain Tolerance is the pressure intensity where the subject stop the stimulation on a push button. Both the pain threshold and tolerance are relevant biomarkers of the pain sensitivity.