1 Introduction

Central sensitization (CS) involves amplified central nervous system (CNS) signaling and several biochemical changes which lead to pain hypersensitivity.^1,2 As opposed to a simplified view of the pain response, where a peripheral stimulus (e.g. object, heat, or injury) causes a severity of pain proportional to the stimulus, CS builds upon this by accounting for the role of the CNS as well. The CNS can alleviate or amplify pain severity as well as increase or decrease the duration of pain perception.¹

Data on the effects of central sensitization in orthopaedics is limited. Regarding surgical outcomes, two studies have examined the use of the Central Sensitization Index (CSI). Coronado et al. found that CSI scores were associated with resilience, anxiety, and negative affect in patients with unilateral shoulder pain, but were not associated with widespread pain sensitivity.³ However, Kondo et al. found that CS significantly affected neck pain and disability in patients undergoing surgery for cervical degenerative disk disease.⁴

While understanding the effect of CS on acute surgical outcomes is clinically relevant, studying the role that it plays in chronic pain is equally important. However, this can be difficult as current research suggests that the effect of CS on pain differs by disease. Mibu et al. found that CSI and widespread pain index (WPI) scores were significantly higher in patients with chronic low back pain compared to patients with knee osteoarthritis despite the fact that there was no difference in subjective pain intensity between groups.⁵ Further, they found that pain-related disability and health related quality of life scores were associated with CSI in both groups. From these findings, they suggested that there be disease-specific CS cutoff scores.^4,5 Somewhat similarly, multiple studies have examined the modulation of pain in patellofemoral pain (PFP). De Albuquerque et al. compared women with PFP to asymptomatic women and found that the PFP group had lower pressure pain thresholds in both the knee and the elbow, but there was no difference in conditioned pain modulation.⁶ Maclachlan et al. noted similar findings, while also noting the prevalence and importance of psychological comorbidities, such as pain-related fear in PFP.⁷

A recent level IV study by Bech et al. found that CSI was associated with the reported levels of postoperative pain after hip arthroscopy.⁸ However, this study only followed up with patients up to 12 weeks from their surgery date. To our knowledge, no higher level prognostic studies have been performed. Therefore, the purpose of this study is to retrospectively investigate the effect that central sensitization has on functional outcome scores of patients who underwent hip arthroscopy at a single institution with a minimum of 2 year follow up. We hypothesize that there will be no relationship between CS and postoperative outcome scores in FAI patients treated with arthroscopic hip surgery.

2 Methods

3 Results

In total, 34 patients were eligible to be included in our study (Table 1) that had an average follow up time of 4.02 ± 0.4 years. Of these 34 patients, 70.6 % (24) were female and 29.4 % (10) were male with an average age of 39.8 ± 25.3. The distribution between right or left hip arthroscopy was 52.9 % (18) and 47.1 % (16), respectively. Patients had an average BMI of 26.3 ± 4.0. Of these 34 patients, 35.3 % (12) underwent a concomitant acetabuloplasty, 76.5 % (26) underwent a labral repair, 67.6 % (23) underwent labral reconstruction, and 2.94 % (1) underwent a femoraplasty. One hundred percent (34) patients had capsular closure, 29.4 % (10) had a synovectomy, and only 5.88 % (2) underwent ligament teres debridement.

When it came to the Central Sensitization Index (CSI), the average score was 30.1 ± 16.0. When evaluating pre-operative patient reported outcome measures, no relationship was seen between VAS pain and PCS scores (correlation value 0.237 and −0.042, respectively). However, MCS and iHOT scores demonstrated moderately negative correlations (−0.607 and −0.573, respectively) when compared to CSI, indicating that those who are more vulnerable to pain hypersensitivity reported lower scores preoperatively. Similar to preoperative scores, postoperative scores had no correlation with VAS pain scores or PCS scores (0.186 and −0.235, respectively), but did have a strong negative correlation with MCS scores (−0.756). Of note, iHOT scores were found to have a weakly positive correlation with CSI postoperatively (0.326). Other scores taken during the postoperative period time such as VAS Satisfaction, PAM 13, and PSEQ showed weakly negative correlations to CSI (−0.300, −0.494, and −0.495, respectively), while BRS Resilience showed a moderately negative correlation to CSI with a value of −0.556 (Table 2).

Pairwise testing showed significant differences between VAS Pain scores (p = 0.002), PCS scores (p = 0.001), and MCS scores (p = 0.024) preoperatively compared to postoperatively. Interestingly, iHOT scores did not demonstrate a significant difference from pre-operatively to post-operatively (p = 0.726) (Table 3).

Patients were stratified by CSI scores into Low CSI, Moderate CSI, and High CSI groups (Table 4). Notable scores that showed significant differences across all 3 groups were preoperative MCS (p < 0.001), postoperative MCS (p = 0.001), and PSEQ2 (p = 0.015). Postoperative VAS pain approached significance but did not meet criteria of p < 0.05 (p = 0.062).

4 Discussion

Central sensitization can occur in patients with chronic hip pain due to various conditions such as labral tears, FAI or osteoarthritis. The persistent nociceptive pain stimuli from the hip joint can lead to the described changes in biochemistry of our patients who display characteristics of Central Sensitization Syndrome (CSS). In 2012, Mayer et al. initially developed the CSI as a self-reporting tool to help identify patients with somatic and emotional complaints that fall within the spectrum of diseases associated with CSS.¹⁰ With the highest score achievable set as 100 (25 prompts worth 4 points each), previous literature amongst chronic pain patients found a “cut off” score of 40 is correlated with 81 % sensitivity of correctly identifying patients with CSS disorders.¹¹

To our knowledge this is the first study of its kind to explore CSI in relationship to both PROMs and BRS with a minimum of 2 year follow up. Within our study, the mean CSI score fell below the 40 point threshold, although 12 patients within our cohort did score above 40 points on the CSI. There is a scarcity of literature exploring the relationship of central sensitization to functional and clinical outcomes in arthroscopy. In a 2021 study, Bech et al. found that numeric pain scores were more closely related to the pain catastrophizing scale. Only after running univariate analysis was the CSI score significantly associated with post-operative pain at 12 weeks.⁸ Patients with CSI scores above 40 had numeric pain scores 1.5 points higher than those with CSI scores under 40. Similarly, in studying total knee arthroplasty and CS, a group in Korea has published various studies exploring the relationship of CS and post-operative pain as well as functional outcomes.^12–14 A 2022 meta-analysis described that patients with CS experienced more severe and persistent pain after surgery than the non-CS group.¹² An earlier 2019 prospective randomized control trial that adding Duloxetine to a post-operative pain control regimen decreased pain in post-operative weeks 2–12, although interestingly had no effect in the first two weeks.¹⁴

Most notably in our pre-operative analysis, MCS and iHOT scores had a negative correlation to CSI scores. Although there is no published literature exploring the relationship of MCS, CSI, and hip arthroscopy, this relationship is not surprising as patients with lower MCS scores may demonstrate mental health challenges or a reduced quality of life prior to surgery which previously has been found to have poorer surgical outcomes.¹⁵ Interestingly the negative relationship between MCS and CSI post operatively became stronger, as compared to iHOT scores which became weakly positive post operatively. The relationship between MCS and CSI is again re-demonstrated in the sub-group analysis pre and post operatively, where those with higher CSI scores had significantly lower MCS scores. One recent study found that patients with low MCS scores pre-operatively were able to reach higher MCS scores post operatively.¹⁶ Both of these results speak to the role a patient's mental state can affect outcomes. By performing a surgical procedure to address the key pain generator of our patient population, this decrease in nociceptive input can lead to a down regulation of the central sensitization pathway, decreasing the mental stress on our patient population and thus may explain the relationship of MCS to CSI. With regards to the relationship between iHOT scores and CSI, there has previously been discussion that CS is due to neuropathologic pain and not musculoskeletal pain.¹⁷ Thus while the surgery may address a patient's limited function due to their musculoskeletal hip pain, it is possible that a central sensitization may still exist and other therapies should be included in treating patients with CS.

There was no correlation between PCS and VAS scores pre-operatively or post operatively. The lack of correlation in the VAS differs from prior literature which used the numeric rating scale,⁸ although this may be explained in the fact that the numeric scale asks patients to only identify the intensity of their pain and not necessarily the character in addition to the intensity of the pain as in the VAS.

The relationship between resilience and CS is complex. CS contributes to increased pain perception and has the potential to affect resilience negatively. Conversely, patients with stronger resilience may be able to adapt to and respond to a chronic pain syndrome better than their counterparts with a weaker resilience. Previous studies across arthroscopy disciplines have demonstrated that patients with lower resilience have lower post-operative PROMs.^18–20 In our study, pre-operative BRS demonstrated a moderately negative correlation to post-operative CSI, affirming that CSI may help to better identify patients who are at a higher risk of developing chronic pain after hip arthroscopy. Future studies with larger patient cohorts and longer-term follow-up are needed in order to understand this complex relationship and ultimately evaluation potentially modifying interventions that can result in improved outcomes in patients with increased CSI.

5 Conclusion

This study showed that increased postoperative CSI scores directly correlated with decreased pre-operative and post-operative MCS scores and worse pre-operative resilience in patients undergoing hip arthroscopy. Recognizing the influence of CS on pain perception and resilience on coping with adversity in the recovery period may guide orthopaedic surgeons in developing comprehensive treatment plans to continue to improve surgical outcomes in hip arthroscopy. Further investigation is needed to determine whether CSI is a useful pre-operative tool for identifying at risk patient populations and which modalities are able to modify any potential risk and improve outcomes after hip arthroscopy.

Guardian/patient's consent

Due to the retrospective nature and IRB approval of this study, patient consent was not needed to be gathered before conducting this research.

Funding sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Institutional Ethical Committee approval

Not applicable.

CRediT authorship contribution statement

Andres R. Perez: Methodology, data collection, Data curation, Writing – original draft, Writing – review & editing. William F. Baker: data collection, Writing – original draft, Writing – review & editing. Neel K. Patel: data analysis, Writing – review & editing. Henson Destine: data collection, Writing – review & editing. Rahul Muchintala: data analysis, Writing – review & editing. Austin Looney: Conceptualization, Supervision, Writing – review & editing. Patrick Szukics: Conceptualization, Supervision, Writing – review & editing. John P. Salvo: study design, Supervision, oversight, Writing – review & editing.