Original articleRegional asymmetry, obesity and gender determines tactile acuity of the knee regions: A cross-sectional study☆
Introduction
Knee pain is a common musculoskeletal disorder encountered in clinical practice, contributing to 17.1 million years of life lived with disability among the world-wide population in 2010 (Woolf and Pfleger, 2003, Cross et al., 2014). A range of pathophysiological manifestations affecting the intra-articular, peri-articular, and extra-articular structures include degenerative, inflammatory, and mechanical processes which may contribute to joint pain (Sokolove and Lepus, 2013). Considering the mismatch between medical imaging findings and clinical signs and symptoms, the etiological mechanisms behind joint pain could be partly explained by central neurological adaptive mechanisms (Staud, 2011). These adaptive mechanisms occur secondary to chronic peripheral nociceptive activity at the receptor, dorsal horn, cortical and sub-cortical areas where the sensory information is processed for perception of pain (Haigh et al., 2003, Arendt-Nielsen et al., 2010, Allen, 2011, Murphy et al., 2012, Lluch et al., 2014).
Chronic nociceptive inputs associated with musculoskeletal conditions are associated with changes to the anatomical representation of the sensory and motor cortices of a specific body region (Wand et al., 2011, Catley et al., 2014). The changes in the cortices are observed in individuals with chronic low back pain, chronic knee pain (knee osteoarthritis), complex regional pain syndromes, and limb amputations (Flor et al., 1997, Grüsser et al., 2001, Pleger et al., 2006, Shanahan et al., 2015). The structural changes in the brain, known as cortical reorganization, adapt to the chronic nociceptive inputs (Flor et al., 1997). Cortical reorganisation is generally quantified by using various electrophysiological and brain imaging techniques for research purposes (Flor et al., 1997, Grüsser et al., 2001, Gwilym et al., 2010, Shanahan et al., 2015). However, clinically feasible methods to quantify such adaptive cortical processes secondary to chronic nociceptive inputs for early intervention is minimal. Recent research suggests that the extent of the cortical representation of a body region can be quantified by assessing the magnitude of two-point discrimination threshold (TPDT) of that body region using a mechanical caliper tool (Moseley, 2008, Moseley et al., 2008, Catley et al., 2013).
Two-point discrimination describes a function of touch that has significant peripheral and cortical neural mechanisms and it is a measure of tactile acuity. Traditionally, the TPDT is used as a measure of cutaneous innervation density (i.e. receptive field size) (Lundborg and Rosen, 2004), which can be influenced by sensory ageing (Koltzenburg et al., 1999, Wickremaratchi and Llewelyn, 2006, Decorps et al., 2014), anthropometric factors including body mass index (BMI) (Falling and Mani, 2016) and body fat ratios (Boles and Givens, 2011), and dermatomal organization (Horner and Dellon, 1994). Recent meta-analysis suggests that there is an increased TPDT in individuals with musculoskeletal pain including chronic knee pain (Catley et al., 2014), where the peripheral nervous system organization is unaltered. This indicates the ability of TPDT to identify altered central somatosensory function in chronic musculoskeletal pain, thereby the TPDT has expanded its clinical utility beyond investigating the integrity of peripheral nerve function (Tamura et al., 2003, Maihöfner and DeCol, 2007).
Recent evidence that investigated the TPDT of the knee joint found significant differences in TPDT in a small, non-age stratified clinical population (chronic knee pain) compared against healthy controls (Stanton et al., 2011, Stanton et al., 2013). This evidence clearly articulates the need for clinical assessment of altered central somatosensory function (e.g. cortical reorganization, warranting the need to embrace interventions targeting such adaptive central processes associated with chronic knee pain (Moseley and Flor, 2012)). However, the statistically significant difference in TPDT estimates was not adjusted for common variables that may have positively or negatively confounded the observed differences. A range of demographic and anthropometric factors and the potential interactions among these factors can influence the sensory acuity of the body region. Literature (Nolan, 1983, Bell-Krotoski et al., 1993, Bowden and McNulty, 2013, Catley et al., 2013) addresses the contribution of demographic (age, gender, side, limb dominance) and anthropometric factors on TPDT estimates in regions of high acuity, however, the underlying mechanisms are not completely understood for regions of lower acuity (e.g knee regions). There is a definitive lack of understanding of how these factors may influence the TPDT over peripheral joints, including knee joints. Establishing an improved understanding of those factors may help inform how one (clinicians/researchers) can normalize cortical representation through targeted interventions designed to address modifiable factors (Moseley and Flor, 2012).
A large body of evidence has established TPDT reference values for areas of high functional acuity, including those of the hand and face (Nolan, 1985, Posnick et al., 1990, Bell-Krotoski et al., 1993). However, only few studies have investigated TPDT over joint regions which are the commonly encountered in musculoskeletal clinical practice (Nolan, 1983, Catley et al., 2013). In addition, for clinical practice, normal reference values are needed in order to help clinicians differentiate normal variability of TPDT against abnormal thresholds in symptomatic populations. To the best of our knowledge, no previous studies have developed reference TPDT values or examined the determinants of knee joint organization. Evidence suggests that sensory deficits exist in the contralateral uninjured side, mediated through several mechanisms including altered central neuronal activity (Donaldson, 1999, Koltzenburg et al., 1999, Shenker et al., 2003, Kelly et al., 2007). This suggests the need for establishing reference values for both knee joints which will help clinicians to differentiate from normal against the abnormal thresholds when assessing for altered central somatosensory function in patients with unilateral or bilateral joint pain. Therefore, the aims of this study are to develop age-stratified TPDT reference values for knee (side and regions), and to investigate the associations of demographic (age, gender, body side, knee regions) and anthropometric variables (body mass index (BMI) and waist-height ratio (WHR)) on TPDT (a measure of tactile acuity).
Section snippets
Study design
This study is a part of the larger cross-sectional research that investigated TPDT estimates of the low back regions (Falling and Mani, 2016). This study was granted ethical approval by the University of X Human Ethics Committee. Written consent to participate in the study was obtained from all participants. TPDT was measured at four sites for each participant, including: medial and lateral regions of both knees. Each site was assessed once, in random testing order generated and counterbalanced
Results
A total of 79 participants across four decades: Group I (18–29 years), Group II (30–39 years), Group III (40–49 years), and Group IV (50–60 years), participated in this study. Anthropometric data was used to calculate BMI and WHR for each participant. Baseline characteristics are summarized in Table 1.
Discussion
The following discussion of study findings is organised by study aims, and therefore includes: TPDT estimates of knee regions in healthy individuals and association of variables with TPDT estimates.
Conclusions
Reference TPDT values for knee regions across four decades have been reported in healthy individuals. Current study results indicated regional differences within the knee, with females demonstrating decreased thresholds of the knee region. The observed linear relationships between body mass, waist circumference and threshold estimates, indicates the role played by obesity in threshold determination. Such influences of anthropometric variables stimulates further interest to investigate the
Conflict of interest
There are no known conflicts of interest associated with this study nor any financial support that could have influenced the outcomes.
Ethical approval
The ethical approval was obtained from the University of Otago Human Ethics Committee.
Funding statement
None.
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The manuscript proposal was approved by the University Human Ethics Committee.