Pressure pain sensitivity in patients with traumatic first-time and recurrent anterior shoulder dislocation: a cross-sectional analysis

2.1 Study design

This was a secondary analysis of cross-sectional data from a randomized controlled trial (RCT) [23]. Reporting was conducted according to the STROBE guidelines for cross-sectional studies [24]. All patients gave informed consent before being enrolled, the study was conducted in accordance with the Helsinki declaration, it was approved by the local Ethics Committee for the Region of Southern Denmark (project ID: S-20140093), and the RCT was registered at ClinicalTrials.gov (NCT02371928).

2.2 Patients

In total, 56 patients were included in the study from three orthopaedic shoulder units in the regions of Southern and Northern Denmark. As this was an explorative analysis of an RCT, no sample size and power calculations were performed for the outcomes presented here. Eligibility criteria were males and females aged 18–39 years with a traumatic ASD (first-time or recurrent event, with a maximum of up to five anterior dislocations verified by patient register and/or subjective evaluation). Furthermore, patients were required to have a minimum of one radiological verified ASD and self-reported shoulder problems in the week prior to assessment for inclusion, e.g. reduced ability to perform specific shoulder movements during sports/leisure activity and/or work. Exclusion criteria included humeral fracture and/or bony Bankart lesion warranting surgery, prior surgery in the affected shoulder, suspected competing diagnosis (e.g. rheumatoid arthritis, cancer, neurological disorders, fibromyalgia, schizophrenia, suicidal tendency, borderline personality disorder or obsessive compulsive disorder), sensory and motor deficits in neck and shoulder, pregnancy, inability to write and speak Danish.

2.3 Procedures

The following parameters were assessed in all patients: (1) assessment of pressure pain sensitivity evaluated by PPTs at the shoulder (local pressure pain sensitivity) and the lower leg (distant pressure pain sensitivity), (2) pain intensity at rest during the previous 24 h, (3) number of ASD, and (4) number of painful body sites on a region-divided body chart [25], [26].

2.4 Assessment of pain intensity, number of painful body sites and dislocation

As part of a larger test battery, data was collected regarding anthropometry (height/weight), pain intensity at rest within the latest 24 h using a Numeric Pain Rating Scale (NPRS; 0–10 score, 10=worst imaginable pain), self-reported shoulder instability using the Western Ontario Shoulder Instability Index (WOSI; 0–2100 better to worse), number of dislocation registered as number of shoulder reductions treated in an orthopaedic unit, clinical test for anterior shoulder instability using apprehension, relocation and surprise (yes/no) tests, and number of painful body sites in which the patient shaded body sites with pain in the previous 24 h on a region-divided body chart (26 sites in total).

2.5 Assessment of pressure pain sensitivity using PPT

The PPTs were measured using a handheld algometer with a 1 cm² probe (Algometer Type II; Somedic AB, Hoerby, Sweden), applied perpendicular to the skin at a constant rate of 30 kPa/s. Pressure was increased until the patient felt the pressure changed from a sense of pressure to pain and pressed a button defining the PPT (rated in kPa). Before the actual test, one or more test experiments were performed on the dorsal aspect of the hand until the patient had understood the purpose of the experiment. The patient was informed that the test was not about examining how much pain they could tolerate but finding the exact transition from pressure to pain. Measurements at the shoulder sites were only performed in the affected/injured side, and none of the patients had bilateral ASD. Exact measurement sites were found with a tape measure and marked with a pen (Fig. 1). Locations tested at the shoulder (local pressure pain sensitivity) were m. trapezius superior, on top of the muscle belly halfway between the spinosi of C7 and lateral border of acromion; m. levator scapula, 2 cm above the superior angle of the scapula (in line with fibers); m. pec. major, 5 cm below the center of the clavicle halfway between the sternoclavicular joint and lateral border of acromion; and m. deltoideus (middle part), 3 cm proximal to the distal humeral insertion. At the lower leg (distant pressure pain sensitivity), m. tibialis anterior on the opposite side of the affected shoulder was tested 5 cm distal to tuberositas tibia and on top of the muscle belly. For m. trapezius superior, the patient was sitting erect on a couch with feet on the ground, arms down, hands resting on thighs. For m. levator scapula, the patient was lying prone with neck in neutral and both arms in neutral with the back of the hand resting on the couch. For the three remaining muscles, the patient was lying supine with neck in neutral supported by a pillow and arms resting in neutral. Both elbows were supported by a small towel to achieve neutral position and avoid stretching mm. pectorales major and deltoideus, and with hands resting on the anterior part of the hips.

Fig. 1:

Locations tested with pressure pain threshold at the shoulder (mm. trapezius superior, levator scapula, pectorales major, deltoideus) and the lower leg (m. tibialis anterior).

2.6 Statistical analysis

Patient characteristics were tested for normality and presented with descriptive statistics. Continuous data was normally distributed (QQ-plots and histograms) and presented as means (SD) and dichotomous data as frequency (%). To assess differences in demographics between patients with first-time or recurrent ASD, unpaired t-tests were used. For dichotomous outcomes, 2-sided Fisher’s exact test was applied.

To assess group differences in outcome measures, preliminary checks were conducted to ensure that there was no violation of the assumptions of normality, linearity, homogeneity of variances, homogeneity of regression slopes, and reliable measurement of the covariate. A two-way ANCOVA was conducted to assess group differences in PPT with site (muscle locations) and ASD status (first-time, recurrent) as factors, adjusting for sex and age. A one-way ANCOVA was conducted to assess group differences in the number of painful body sites with ASD (first-time, recurrent) as factor, adjusting for sex and age. Due to equal variance but unequal sample size, Tukey–Kramer was used as a post hoc test in case of significant ANCOVA factors or interactions.

An unpaired t-test was used to assess between-sex differences in PPTs. Pearson’s product-moment correlations were used to assess the relationship between shoulder PPTs, lower leg PPTs, painful body sites, and pain intensity, while correlations with the number of dislocation were conducted using Spearman’s ρ. The strength of association was defined as follows: 0.1<|r|<0.3 small correlation, 0.3<|r|<0.5 medium/moderate correlation, and |r|>0.5 large/strong correlation [27]. All statistical analyses were performed using STATA (StataCorp, 2015, Stata Statistical Software: Release 15.1, College Station, TX, USA: StataCorp LP.), and p-values of less than 0.05 were considered significant.

4.1 Strength and limitations

Due to the exploratory nature of the analysis, the results must be interpreted with caution. Firstly, it is important to acknowledge that since the sample size was determined based on the primary RCT, the non-significant findings of this analysis could merely be a result of a type II error. However, the reported differences were small, indicating that even with a larger sample size any potential differences would not be clinically relevant. The current measurements were performed only 3–6 weeks after the latest ASD, and precise symptom duration was not collected. As such, we cannot rule out that the findings are explained by the painful inflammatory process that follows after an acute traumatic injury. The data is also limited, because PPT-measurement is just one factor when assessing sensitization in painful conditions, where other measurements such as suprathreshold heat pain responses and psychological factors such as fear-avoidance could provide useful knowledge about potential changes and adaptations in the pain system. The strengths of the study are that the analysis was built upon data from an RCT thereby strengthening the standardization of testing procedures, and the fact that data was collected in clinical practice with a clinically applicable setup for measurement of PPT.

4.2 Conclusion

This study demonstrated no significant differences in local or distant pressure pain sensitivity or number of painful body sites between patients with first-time and recurrent traumatic ASD. Females had lower PPTs than males, and a strong correlation was found between PPTs in the shoulder and at the lower leg.

4.3 Implications

Patients with first-time and recurrent ASD seem to have similar pressure pain sensitivity with lower PPTs compared to existing normative data [16], [18], [31], [32], [33], suggesting that it is relevant to understand and evaluate the status of the pain system in these patients to prevent triggering or worsening their symptoms. However, it remains unanswered how these changes affect the patients’ ability to undergo rehabilitation, their symptom response and long-term shoulder function.