Radiological Comparison of Femoral Neck–Shaft Angle in Piriformis Syndrome: A Case–Control Study

Abstract

Introduction

Variations in osseous, neural and muscular anatomical structures in the gluteal region have been investigated for their role in causing deep gluteal pain syndromes including piriformis syndrome. This study aimed to radiologically determine whether the femoral neck–shaft angle (NSA) in piriformis syndrome differs from that in a healthy population.

Methods

Two groups of participants comprising 23 piriformis syndrome patients and 22 healthy controls were included in the present study. Piriformis syndrome was diagnosed based on clinical findings and the diagnosis was confirmed by intramuscular local anaesthetic injection. Femoral NSAs were measured from the anteroposterior pelvic radiographs and compared between the control and piriformis syndrome-affected groups.

Results

The age, height, body weight, body mass index and gender distribution differences between the two groups (control and piriformis) were not statistically significant. The femur NSA was significantly lower in the piriformis group than in the control group. The receiver operating characteristic analysis revealed that the area under the curve, sensitivity and specificity for predicting the risk of piriformis syndrome (PS) were 0.946%, 100% and 86.36%, respectively, at an NSA cut-off of 127°.

Conclusions

Reduced femoral NSA is related to PS. However, studies with a larger study sample are needed to further substantiate this finding.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43465-022-00736-y.

Introductıon

Piriformis syndrome (PS) is a neuromuscular disorder caused by the compression of the sciatic nerve and other anatomical structures that pass under the piriformis muscle (PM). Conventionally, the term PS has been used as an umbrella term to describe posterior hip pain due to nerve compression. Nowadays, it is recognized as a subtype of deep gluteal pain syndrome (DGS) together with gemelli-obturator internus syndrome, ischiofemoral impingement syndrome and proximal hamstring syndrome [1–3]. DGS is defined as non-discogenic nerve compression caused by any structure in the deep gluteal space, while PS is caused by primary and secondary pathologies of PM [1, 2]. PS is believed to be responsible for 0.3–6% of all cases of lower back pain and/or sciatica. It is an important yet overlooked differential diagnosis in the workup of lower back pain and sciatica [4]. Gluteal trauma, anatomic variations of the sciatic nerve relating to PM, PM hypertrophy or spasm, prolonged sitting, space-occupying lesions and hip interventions are believed to cause PS [5].

Anatomical proximity to the PM facilitates the comprehension of the pathophysiology of sciatic nerve entrapment in PS. Conditions that cause PM spasms, shortening or excessive contraction may compress the sciatic nerve beneath the muscle, resulting in PS [6]. A change in the relationship between the anatomical structures caused by biomechanical constraints may result in the reduction of the supra-piriform and the infra-piriform passageways and nerve entrapments [7]. However, past studies on altered skeletal morphology, which may result in a reduction of the passageways in PS, have been limited to cadaveric studies [8]. To the best of our knowledge, no study has yet radiologically examined the femoral neck–shaft angle (NSA) in PS. This study aimed to radiologically determine whether the femoral NSA in PS is different from that in a healthy population.

Methods

This was a case–control study. Ethics approval was obtained by the institutional clinical research ethics committee of the local hospital. Written consent was obtained before the examination from all subjects. From 1 April 2018 to 29 March 2020, we enrolled patients from the physical medicine and rehabilitation outpatient clinic who presented with a complaint of gluteal pain with or without radiant pain to the posterior thigh.

Sample Size

Femoral NSA was selected as the primary outcome variable to determine the number of subjects enrolled in this study. Group sample sizes of 20 PS and 20 control subjects achieved 81% power for the detection of a 5% difference between the two groups, indicating a significance level (alpha) of 0.05 on a two-sided two-sample t test. The double-block randomisation method was applied for the selection of the control group subjects included in the study, while the Random Allocation Software package programme was used for the application [9].

Participants

The inclusion criteria for the PS group were patients presenting with gluteal pain with or without radiation to the ipsilateral posterior thigh, the presence of one or more positive PS physical examinations, and at least 50% reduction in pain after the intramuscular LA injection. The inclusion in the group of healthy volunteers required a normal history and physical examination for PS.

The exclusion criteria included patients presenting with a neurologic deficit, limb length discrepancy, lower back pain, sacroiliac, hip pathology (inflammatory, degenerative, or mechanic), surgical history at the lumbar and hip regions, being in the gestational period, having a contraindication to LA and an active psychiatric illness.

Anamnesis and History

Pain duration, the side of gluteal pain, factors leading to pain alleviation, initiating event, trauma history and other symptoms including lower back pain and leg paraesthesia were recorded.

Physical Examination

A complete examination of the lower back, pelvis, buttock, hip, sacroiliac joint and lower extremities included inspection, range of motion, palpation, muscle strength, sensation and special tests depending on the patient’s presenting symptoms, which were conducted by an expert physiatrist [10]. Deep gluteal palpation, SLR test and special PS manoeuvres were included in the PS evaluation (Table 1; Fig. 1) [10]. After each manoeuvre, we waited for 1 min in the test position to elicit any symptoms. If any familiar complaints were reproduced, the tests were recorded as positive.

Table 1.

Special tests performed for PS evaluation

Freiberg sign	Examiner forces the lower extremity to internal rotation and adduction while the patient is in the supine position and the hip and knee are extended (Fig. 1a) [11]
Pace test	The patient sits on the examination table, legs hanging over the edge of examination bed and asked to abduct his legs against the resistance (Fig. 1b) [12]
Beatty test	The patient lies on the asymptomatic side in a lateral decubitus position. On the symptomatic side, the hip and knee flexed. The patient is asked to perform lateral rotation and hip abduction movements against manual resistance (Fig. 1c) [13]
Flexion adduction internal rotation test	With the patient in the supine position, knee and hip flexed, forced adduction and internal rotation of the lower extremity is performed (Fig. 1d) [12]
Heel contralateral knee manoeuvre	In the hook lying position, the patient places the heel of the foot of the symptomatic side above the contralateral knee, and examiner flexes the contralateral hip (Fig. 1e) [7]

Fig. 1.

Special tests for piriformis syndrome. A Freiberg sign. B Pace sign. C Beatty manoeuvre. D Flexion Adduction Internal Rotation (FAIR) manoeuvre. E Heel Contralateral Knee (HCLK) manoeuvre

The control participants were collected from the Picture Archiving Communication Systems database from April 2018 to March 2020. Radiographs of the patients who had undergone pelvic radiographs in the supine position with an internal rotation of the feet of 15°–20° to exclude pelvic fractures in emergency care or outpatient clinics of the hospital were evaluated. Only radiographs reported by the radiologists as normal were included in the study. Data on subject age, gender, history and symptoms were collected. The control participants had no current or past history of the lower back, hip, gluteal pain and sciatica. The clinical status of the patients was confirmed on their hospital visit. All participants provided their written informed consent.

Biochemical Analysis

Full blood count, ESR, CRP, urine analysis, renal function tests and coagulation tests were performed to exclude the presence of inflammatory and infectious diseases and determine the presence of any contraindications to LA injection if present.

Imaging Studies

Lumbar MRI was performed for excluding radiculopathy or spinal stenosis, and in the presence of red flag signs and atypical pain pelvic, the MRI images of the pelvis were obtained. All studies were performed on the data of MRN conducted on the 1.5-T MRI 450w system (GE Healthcare, Milwaukee, WI, USA).

US-Guided Diagnostic Injection

The US-guided diagnostic injection was performed to confirm the diagnosis of PS. The buttock region was scanned with a 6-to-15–MHz curvilinear array transducer, with one of the edges of the transducer placed on the great trochanter and the other directed through the lateral border of the sacrum. In this position, the transducer was kept parallel to the fibres of the PM (Fig. 2). PM was visualised under the gluteus maximus muscle and 4-cc of 2% lidocaine was injected into the PM fibres [14]. A total of 23 participants with a ≥ 50% significant reduction in pain were included in the PS group (Fig. 3) [15].

Fig. 2 .

Imaging of the piriformis muscle by ultrasonography. Triangular piriformis muscle can be seen under the fibres of the gluteus maximus between the trochanter major and the sacrum

Fig. 3.

Steps in participant selection for the PS group

Radiographs

AP pelvic radiographs of the participants in the supine position were taken with an internal rotation of the feet by 15°–20° using a standard radiographic protocol. Femoral NSA was measured on the AP pelvic radiographs using a single handheld 360° goniometer (Fig. 4). The arms of the goniometer were aligned along the femoral head–neck axis and the longitudinal axis of the femoral shaft [16]. The same goniometer was used for all measurements. Femoral NSA measurements—the primary outcome variable of the study—were repeated twice 1-week apart by two senior experts who were blinded to the patient’s information. These parameters were analysed and compared between the PS and control groups statistically.

Fig. 4.

Measurement of femur neck–shaft angle. The arms of the goniometer were aligned along the femoral head–neck axis and the longitudinal axis of the femoral shaft. α: femur neck–shaft angle

Data Analysis

Statistical analyses were performed with the SPSS v21 (SPSS Inc., Chicago, IL, USA) package. Quantitative data were summarised by mean ± standard deviation and median (minimum–maximum), while the qualitative data were depicted as frequency and percentage. The Shapiro–Wilk and Kolmogorov–Smirnov tests were performed to assess the normal distribution of the data, while the homogeneities of variances were tested by the Levene test. Student’s t test or Mann–Whitney U test for continuous variables and Chi-square and/or Fisher’s exact test for categorical variables were employed to evaluate the differences between the groups. Receiver operating characteristic (ROC) curves were applied to describe the performance of diagnostics value of the femoral NSA in detecting the presence of PS. The area under the corresponding curves was calculated and compared as described by Hanley and McNeil [17]. Inter- and intra-observer agreement was measured using the intra-class correlation coefficient using the one-way and two-way random models for consistency and the absolute agreement options for single and average measurements.

Results

A total of 45 participants (23 patients and 22 controls) were included in the study. Ages in years in the PS group varied from 28 to 64 with a mean of 41.65 while that in the control group varied from 17 to 57 with a mean of 35.5. The mean age of all individuals was 38.64 ± 10.74 (17–64) years. While the PS group consisted of 18 (78.26%) women and 5 (21.74%) men, the control group consisted of 19 (86.36%) women and 3 (13.64%) men. The groups did not show any statistically significant difference in terms of age, height, body weight, body mass index (Table 2) and gender distribution (p = 0.699).

Table 2.

Demographics and measurements of the study groups

	Control		PS		p
	Mean ± SD	Median (min.–max.)	Mean ± SD	Median (min.–max.)
Age	35.5 ± 10.51	35 (17–57)	41.65 ± 10.3	39 (28–64)	0.054
Length	165.09 ± 7.95	166 (150–180)	165.57 ± 10.24	165 (149–187)	0.863
Weight	65.05 ± 9.17	64.5 (52–85)	70.61 ± 16.08	67 (53–105)	0.439
BMI	23.97 ± 3.74	23.01 (19.13–30.84)	25.65 ± 4.62	23.8 (19.47–35.13)	0.188
Femur NSA	128.89 ± 2.39	129.13 (121.75–131.5)	123.17 ± 3.02	123.75 (114.5–127)	< 0.001*

*p values from the non-parametric Mann–Whitney U test, all others from Student's t test

The femoral NSA was significantly lower in the PS group than in the control group (p < 0.001). The intra-class correlation coefficients between the measurements of the first and second experts (intra-rater) (one-way random-absolute agreement), were 0.90 (0.82–0.94) and 0.98 (0.96–0.98), respectively. On the other hand, inter-rater reliability was calculated (two-way random—consistency) using the average values and was calculated as 0.98 (0.97–0.99). The potential of the femoral NSA variable to determine PS was evaluated by ROC (area under the curve) analysis. Accordingly, AUC was found to be 0.946 (0.834–0.991) and was statistically significant (p < 0.001). The cut-off point determined according to the Youden index was found to be 127, which corresponds to 100 sensitivity and 86.36 specificity points.

In the PS group, pain duration ranged between 1 month and 5 years. None of the patients experienced bilateral pain. Moreover, 65.12% of the patients could not identify an inciting event. Of the 23 patients, 11 (47.83%) experienced pain on the right side whereas 12 (52.71%) on the left side. Prolonged sitting was the most reported factor in triggering the pain (52.17%). A total of 5 patients (21.74%) had a history of trauma while patients experienced varied symptoms like lower back pain (n = 3; 13.04%), ipsilateral leg pain (n = 16; 69.57%) and ipsilateral leg paraesthesia (n = 9; 39.13%) (Table 3). The clinical manoeuvre tests designed to provoke pain were systematically performed in the PS group. Deep gluteal palpation triggered pain in all the patients. Freiberg sign, Flexion Adduction Internal Rotation (FAIR) manoeuvre, Pace sign, Beatty test and Heel Contralateral Knee (HCLK) manoeuvre were positive in 10, 17, 9, 13 and 20 of the patients, respectively (Table 3). SLR test was positive in only one of the patients (4.35%).

Table 3.

Findings of history and physical examinations of the subjects in the PS group

	Frequency, n (%)
Side
Right	11 (47.83)
Left	12 (52.17)
Factors alleviating pain1
Walking	6 (26.09)
Prolonged lying	1 (4.35)
Prolonged standing	8 (34.78)
Prolonged sitting	12 (52.17)
Initiating event1
Sportive activities	6 (26.09)
Blunt trauma over buttock	0 (0.00)
Prolonged sitting	4 (17.39)
Unidentified	15 (65.22)
Trauma history	5 (21.74)
Low back pain	3 (13.04)
Radiating leg pain	16 (69.57)
Leg paraesthesia	9 (39.13)
Clinical examination
Pain with deep gluteal palpation	23 (100.00)
SLR
Negative	22 (95.65)
Positive	1 (4.35)
Flexion adduction internal rotation test
Negative	6 (26.09)
Positive	17 (73.91)
Pace sign
Negative	14 (60.87)
Positive	9 (39.13)
Freiberg sign
Negative	13 (56.52)
Positive	10 (43.48)
HCKL Manoeuvre
Negative	3 (13.04)
Positive	20 (86.96)
Beatty test
Negative	10 (43.48)
Positive	13 (56.52)

¹Patients with more than one factor

Discussion

PS is a neuromuscular disorder caused by the compression of sciatic nerve and other anatomical structures that pass under the PM. The majority of cases occur in middle-aged patients with a reported female to male ratio of 6:1 [5]. In this study, the mean age of the PS group was 41 years which was in agreement with earlier reports, however, the ratio of female to male was found to be 4:1. This difference in ratio might be due to the limited number of subjects in the current study. In most of the patients, the exact cause could not be identified. Previous gluteal trauma was reported as the most common cause in earlier studies [5]. The most common cause reported by participants in this study was sports/physical activities but none of them could be definitively connected to gluteal trauma, as the patients could not recall any critical injuries.

According to Hopayian et al. [18], the most common signs of PS were buttock pain, pain aggravated on sitting, external tenderness near the sciatic notch, pain on PS manoeuvres and limitation of SLR. Similarly, in this study the most common symptoms were buttock pain, pain with deep gluteal palpation, pain on PS-specific manoeuvres, pain aggravated on sitting and ipsilateral leg pain. Paraesthesia was accompanied by ipsilateral leg pain in several patients. Lower back pain was present in three patients. Gluteal pain, ipsilateral leg pain, paraesthesia and lower back pain decreased by > 50% after PM anaesthetic injection. Lower back pain does not rule out the diagnosis of PS [19].

Several physical examination manoeuvres have been described in the literature. Different sensitivity and specificity values for physical examination manoeuvres have been reported; however, none of these can be considered to be ultimate for the diagnosis [10]. In this study, Freiberg sign, FAIR manoeuvre, Pace sign, Beatty test and HCLK manoeuvre were employed for the evaluation of PS. HCLK manoeuvre test showed the highest positivity rate of 86.96%. Although physical examination manoeuvres are helpful for patient evaluation, they cannot be used for definitive diagnosis. The diagnosis of PS should be confirmed with an anaesthetic injection. In this study, three patients with high clinical suspicion provocative test for PS failed the LA injection. In these patients, coxarthrosis, endometrioma and small acetabular fracture were detected during the diagnostic work-up and they were hence excluded from the study. Intramuscular injection of LA in PS is considered both diagnostic and therapeutic. Out of the 23 PS patients, 9 were relieved with a single injection, 5 required additional physical therapy program and 9 required repeated injections at 6–12-month intervals.

In this study, the SLR test was found positive in one patient; however, this positive could not be reproduced following LA injection. According to Michel et al., a positive SLR test exclude the diagnosis of PS [7]. In a systematic review by Hopayian et al., SLR was positive in 31% of the PS patients [20]. Our results support the earlier finding that the SLR test may be positive in PS in case of sciatic nerve irritation [21].

Recent systemic reviews and meta-analyses showed that anatomical variations of the sciatic nerve are related to PM and may play a role in the aetiology of PS [22–24]. Normally, the sciatic nerve passes under the PM with a single trunk. In almost 10% of the population, the sciatic nerve follows a different course, branches proximally and pierces the PM. The change in the path or branching of the sciatic nerve and the decrease in the space through which it exits from the pelvis can cause nerve compression, leading to PS [22]. Although there are many studies on anatomical variations of the sciatic nerve related to PM, the variations in the bony morphology of the pelvis and PS aetiology have not been adequately investigated. Reduced femoral NSA may also result in the alteration of passageways or muscle stretch and lead to nerve entrapments.

Fearon et al. reported that reduced femoral NSA is a risk factor for refractory trochanteric bursitis pain syndrome [25]. They asserted that the shortening of the abductor lever arm increases the loading of the gluteus medius muscle to cause tendinopathy [25]. Considering that PM is one of the muscles contributing to the abductor hip muscle strength, the load on PM may be increased with the same mechanism used for decreasing the NSA. Another extra-articular cause of gluteal and hip pain in which changes in the hip biomechanics have been discussed in the literature is the ischiofemoral impingement (IFI) syndrome. The main pathology in IFI is the narrowing of the ischiofemoral space (IFS). IFS is the shortest distance between the lateral cortex of the IT and the medial cortex of the lesser trochanter [26, 27]. Several studies have reported that patients with IFI have increased femoral NSA [28]. Increase in the femoral NSA causes a decrease or narrowing of the IFS [27]. When the IFS decreases, the quadratus femoris muscle comes at risk of impingement. Similarly, a decrease in the femoral NSA may cause a decrease or narrowing of the space for PM, possibly resulting in a shortening of the PM and a predisposition to PS.

Study Limitations

The present study has the following limitations. First, the main limitation of this study is the small sample size due to the relative rarity of this condition. Second, femoral NSA measurements were taken on AP pelvic radiographs. Third, although not statistically significant, the mean age of the PS group was slightly lower than that of the control group. Future research involving measurements with three-dimensional imaging methods and a larger study sample could provide more accurate analyses and a better understanding of the role of femoral morphology in PS. Despite these limitations, this is the first study to document that reduced femoral NSA is associated with PS.

Conclusion

PS is related to reduced femoral NSA. Owing to its limitations, the current study can be considered as a pilot study to investigate whether femoral NSA is a risk factor for PS. However, further studies are needed to validate the relationship between femoral morphology and PS.

Supplementary Information

Below is the link to the electronic supplementary material.

Author Contributions

Conceived and designed the analysis: FUO, İA, and GGG. Data collection: GGG, İA, and FÜÖ. Contributed data/analysis tools: GGG and FÜÖ. Performed the analysis: GGG, İA, and FÜÖ. Wrote the paper: GGG and İA. Revision of the paper: İA and FUO.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

Declarations

Conflict of interest

The authors declare that there is no conflict of interest.

Ethical Standard Statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.

Informed Consent

For this type of study, informed consent is not required.