Predictors of pain after knee joint replacement

A number of clinical, psychological, neurophysiological and genetic variables will be collected preoperatively and perioperatively to ascertain which are significant, independent predictors of persistent knee pain 6 months after undergoing primary total knee joint arthroplasty. The predictor variables collected will be as follows: Preoperative Predictors (Assessed 1-3 weeks before surgery) 1. Preoperative Knee Pain - Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) Pain subscale 2. Comorbid Pain - Pain > 2/10 on numerical rating scale (0 = no pain; 10 = extreme pain) that has been present on at least a weekly basis for 1 month or longer. Count number of anatomical locations apart from index knee where this is true (e.g. n = 3). 3. Pain expectation - measured using the following question "Please indicate the level of pain you think you will experience in your operated knee 6 months after your surgery?" 0-10 Numerical rating scale (0= no pain; 10 = extreme pain). 4. Pain Catastrophising - measured using the Pain Catastrophsing Scale 5. Depression - measured using the Beck depression inventory 6. Anxiety - measured using the state trait anxiety inventory 7. Temporal summation of mechanical pain - assessed through the repetition of 10 standardised punctuate stimuli at a frequency of 1 Hz, applied to an area of the skin 1cm lateral to the midline of the patella using a 225.1g Von Frey filament. Patients will be asked to rate the pain intensity of the first and last stimuli on a 0-10 numerical rating scale (0 = no pain; 10 = extreme pain). The difference in pain intensity between the last and first stimuli will be calculated to indicate the magnitude of temporal summation. 8. Conditioned Pain Modulation Efficacy - Pressure pain threshold at the index knee will be assessed alone (test stimulus) and while the contralateral hand is sumberged in cold water (conditioning stimulus). The difference in pressure pain thresholds (kPA; mean of 3 measures) with and without the conditioning stimulus will be used as a measure of conditioned pain modulation efficacy. 9. Single nucleotide polymorphisms for the Catechol-O-methyl transferase (COMT), mu opiod receptor (OPRM-1), tumor necrosis factor alpha (tnf-alpha); interleukin-1 (IL-1) and interleukin-6 (IL-6) genes - obtained through a single blood sample that will be taken at the same time as routine preoperative bloods. Standard DNA extraction, polymerase chain reaction, microarray and detection methods will be used to identify single nucleotide polymorphisms. Perioperative Predictors (assessed Day 1-3 post op) 1. Acute pain at rest - Pain scored on a 0-10 numerical rating scale (0 = no pain, 10 = extreme pain) while lying in a supine position with knee extended. Sum of pain scores on post op days 1-3 taken as acute pain at rest. 2. Acute pain on movement - Pain scored 0-10 numerical rating scale (0 = no pain, 10 = extreme pain) after moving knee into maximum flexion and returning to extended position. Sum of pain scores on post op days 1-3 taken as acute pain on movement.