Femoral Neck Impingement Cysts Treated With Biocomposite Implants in the Setting of Femoroacetabular Impingement Show Improvement in Patient-Reported Outcomes at a Minimum 2-Year Follow-Up

Purpose

To assess minimum 2-year outcomes in patients treated with biocomposite anchors for impingement cysts of the femoral neck during hip arthroscopy for femoroacetabular impingement.

Methods

Patients aged 18 to 65 years who underwent treatment of a femoral neck impingement cyst with placement of a suture anchor by the senior surgeon between January 1, 2007, and December 1, 2019, and were eligible for a minimum 2-year follow-up were identified. Demographic, surgical, and patient-reported outcomes—including Hip Outcome Score–Activities of Daily Living (HOS-ADL), Hip Outcome Score–Sport (HOS-SSS), modified Harris Hip Score (mHHS), patient satisfaction, and Tegner Activity Scale—were retrospectively reviewed from the senior surgeon’s prospectively collected database. Complications, including femoral neck fracture, need for revision, or conversion to total hip arthroplasty, were assessed from patient charts and postoperative follow-up surveys. Postoperative magnetic resonance images were assessed for suture anchor integration and presence of a residual cyst.

Results

Fifty patients (50 hips) met inclusion criteria, with a median age at surgery of 45 (range, 19-63) years. Minimum 2-year follow-up was obtained for 43 of 50 (86%) patients, with a mean follow-up time of 3.4 ± 1.4 years. One patient reported conversion to total hip arthroplasty, and no additional patients reported revision surgery or postoperative femoral neck fractures. HOS-ADL, HOS-SSS, mHHS, Western Ontario and McMaster Universities Osteoarthritis Index, and Short Form 12 Physical Component Summary scores significantly improved after hip arthroscopy ( P <.001), with most patients attaining minimum clinically significant difference and patient acceptable symptom state for HOS-ADL, HOS-SSS, and mHHS. Notably, the median postoperative patient satisfaction score was 9.0 of 10 (range, 1-10).

Conclusions

Hips with femoral neck impingement cysts treated with biocomposite suture anchors showed excellent patient-reported outcomes and high survivorship at a minimum 2-year follow-up, with most patients achieving mHHS minimum clinically significant difference and patient acceptable symptom state.

Level of Evidence

Level IV, retrospective therapeutic case series.

Cam-type femoroacetabular impingement (FAI) is often characterized by a bony deformity, causing decreased femoral head-to-neck offset, and it is commonly addressed with osteoplasty during hip arthroscopy to restore the anatomic concavity of the femoral head-neck junction. Although the exact cause is unknown, femoral neck impingement cysts may be identified at the site of impingement during cam resection. These radiolucent, round, subcortical cysts are filled with synovial fluid and surrounded by a thin sclerotic rim and overlying cortex. They are most often present in the anterosuperior femoral head-neck junction and can range in size from a few millimeters to over a centimeter in width or depth ( Fig 1 ). Subcortical femoral head-neck junction cysts were first described by Pitt et al., using the term “herniation pits,” and taking on the colloquial term “Pitt’s pits.” These bony defects were originally thought to be the result of mechanical stress and herniation involving the anterior joint capsule and overlying musculature. More recently, a growing body of literature has associated these cysts with the presence of FAI ^,^, and, more specifically, cam-type FAI.

Image, AltText currently not available — Fig 1

Treatment of impingement cysts varies, and the literature suggests treatment options such as no intervention, physical therapy, activity modifications, intra-articular corticosteroid injections, or surgical decompression. ^,^, Additionally, some studies have reported adequate outcomes following treatment with bone grafting with both allograft and iliac crest autografts. ^, Despite these findings, an established evidence-based treatment option for femoral neck impingement cysts lacks consensus. This is of particular concern as isolated decompression of these cysts may lead to significant complications, such as biomechanical compromise and an increased risk of femoral neck fracture given their location, focal osseous disruption, tendency to increase in size over time, and cortical discontinuity following burr perforation in cam osteoplasty. ^,^,

The recognition of this pathology incidentally corresponded with the development of biocompatible suture anchor fixation devices. Suture anchor technology has followed the aims of maximizing pullout strength and reducing the potential for iatrogenic complications, such as anchor migration and cartilage damage. ^, To better achieve these aims, a focus of advancement in suture anchor design has been the development of polymers that are immunologically inert and more absorbable, as well as more stimulating of the formation of healthy bone. Purely biodegradable suture anchors have the potential to degrade too quickly, leaving a bony defect and triggering a granulomatous cystic reaction, and they have been shown in the proximal humerus to be replaced not by bone but by calcified fibrotic tissue. These drawbacks led to the development of biocomposite suture anchors that contain a balance of a varying absorbable polymer and a longer-lasting bioceramic component to both provide more lasting stability and promote bone formation.

While arthroscopic treatment of cysts of the acetabulum and femoral head using a bone graft substitute in the setting of FAI has been described, studies are limited in assessing the use of biocomposite suture anchors as a treatment for cysts of the proximal femur. The purpose of the present study was to assess minimum 2-year outcomes in patients treated with biocomposite anchors for impingement cysts of the femoral neck during concomitant hip arthroscopy for FAI. It is hypothesized that hips with impingement cysts treated with biocomposite suture anchors will show improved patient-reported outcomes postoperatively from baseline and high survivorship at a minimum 2-year follow-up.

Methods

The present study received institutional review board approval at the senior author’s (M.J.P.) institution (Vail Health Institutional Review Board). All patients who underwent primary hip arthroscopy for the treatment of FAI by the senior surgeon between January 2007 and December 2019 were retrospectively reviewed from the senior surgeon’s prospectively collected database by members of the study staff. Patients who underwent hip arthroscopy for treatment of FAI and underwent treatment of a femoral neck impingement cyst with placement of a suture anchor by the senior surgeon were identified and included for further analysis. Patients aged 18 to 65 years were included if they had an impingement cyst and were able to consent for themselves. Exclusion criteria included lack of an impingement cyst, international residents who were unable to participate in follow-up due to insufficient contact information, and a history of traumatic hip dislocation, Legg-Calve-Perthes disease, or avascular necrosis of the femoral head ( Fig 1 ).

Demographic and radiographic data were retrospectively reviewed, including patient age, sex, α angle, and Tönnis grade. Notably, each radiographic parameter was measured by a member of the senior author’s clinical team. The presence of cam or pincer impingement, as well as surgical decompression at the femoral head-neck junction, acetabular rim, and anteroinferior iliac spine region, was retrieved from postoperative reports. Complications, including femoral neck fracture, need for revision, or conversion to total hip arthroplasty, were assessed from patient charts and postoperative follow-up surveys. Patient-reported outcome (PRO) scores were elicited pre- and postoperatively and included modified Harris Hip Score (mHHS), Hip Outcome Score (HOS), Western Ontario and McMaster Universities Osteoarthritic Index, the 12-item Short Form 2 (SF-12), Tegner activity level, and patient satisfaction.

Additionally, the charts of included patients were reviewed for the presence of pre- and/or postoperative magnetic resonance imaging (MRI). The preoperative images were assessed for the presence of a femoral neck impingement cyst. When available, postoperative MRIs were evaluated to assess the bone response to the inserted suture anchors (anchor grade). Size and location of cysts, anchor types, anchor sizes, and additional related operative data were also recorded.

Surgical Technique

After standard hip arthroscopy setup and positioning of the patient in the supine position with placement of a perineal post, standard anterolateral and mid-anterior portals are established, which have been described previously by the study team. After intraoperative assessment, the central compartment is addressed first. The acetabular rim is exposed, and acetabuloplasty is performed with a curved 4.5-mm burr for future labral repair. The amount of bone to be resected is predetermined based on preoperative radiographic evaluation. Subspinal impingement is also addressed if present. After completing labral management and addressing central compartment pathology, traction is released, and the peripheral compartment is addressed with the hip flexed at 45°. Osteoplasty of the femoral neck is performed with an arthroscopic burr after dynamic evaluation of the hip is performed to identify the location of cam morphology and impingement with the labrum. After decortication of the cam, the impingement cyst is typically encountered ( Figs 2 , 3 ). Patients in whom there was a resulting defect in the subchondral bone at the site of the impingement cyst following completion of the osteoplasty procedure are indicated for cyst treatment with anchor placement. To avoid a stress riser at this site, our preferred technique is to fill the cyst with an appropriately sized anchor. To do so, a straight or curved curette and burr are used to decompress the cyst ( Fig 3 ). This also helps to determine the direction and extent of the cyst. The diameter of the cyst is measured, and a tap is used to guide the insertion of the anchor, followed by anchor placement, with a goal of using the anchor to both occupy the defect and provide a structure for surrounding tissue ( Fig 3 ). For all patients in the study cohort, we used either a TwinFix Ultra hydroxyapatite biocomposite suture anchor (Smith & Nephew) or an OsteoRaptor biocomposite suture anchor (Smith & Nephew), both composed of poly(L-lactate) (PLL), a slowly absorbed polyester that can stimulate collagen formation, and hydroxyapatite (HA). If the defect is too large for 1 anchor, then 2 may be placed in a side-by-side fashion to allow for better interference fit of the anchors. The sutures are removed, and the anchor is meticulously contoured with an arthroscopic burr until it flushes with the surrounding bony surface ( Fig 3 ). Decompression and contouring are continued until appropriate resection has been achieved, when a dynamic examination is performed intraoperatively to reproduce motions at risk, and complete relief of impingement with preservation of the labral seal in all directions is observed. Full cam decompression and confirmation with intraoperative dynamic examination are performed as described in previous technical notes. Complete contouring of the biocomposite anchor is again confirmed before closure of the capsule. Lastly, capsular closure is performed to preserve the stability of the hip joint. In this case, 2 No. 2 Vicryl (Ethicon) sutures are used, each passed through the capsular leaflets in a double-limb fashion with a suture passer and secured with a Quebec City Slider, followed by 5 half-hitch knots.

Rehabilitation

Our postoperative protocol includes placement of a postoperative hip brace and antirotational boots and restriction to 50% flat-footed weightbearing for 10 to 21 days (unless a microfracture is performed, in which case, patients are 20 pounds flat-footed weightbearing for 6 weeks). To reduce the formation of adhesions, we prescribe continuous passive motion for at least 4 weeks, 6 to 8 hours per day, with passive range-of-motion exercises. Stationary biking without restriction is initiated within the first few days after surgery. Hip external rotation and extension are limited to 3 weeks. After the patient has progressed from passive motion through active motion to specific strength exercise milestones, they undergo the Vail Hip Sport Test to aid in determining their readiness to return to their previous activities.

Statistical Analysis

Patient-reported outcomes were reported with means and standard deviations. Cyst size was quantified as the volume of the rectangular prism or ellipsoid that contained the cyst based on coronal, sagittal, and axial diameter measurements. The Wilcoxon signed-rank test was used to test for statistically significant improvements from baseline to a minimum of 2 years postoperatively. Correlation between continuous variables was assessed using Spearman’s rho. P values less than.05 were deemed statistically significant. Given the fixed sample size of 42 patients with known 2-year PRO measure scores and assuming a 2-tailed paired comparison of central tendency with an α level of 0.05, 80% statistical power was achieved to detect an effect size of Cohen’s d = 0.46. Changes between pre- and postoperative PROs were calculated to determine clinically significant outcome improvements utilizing the minimum clinically significant difference (MCID) and patient acceptable symptom state (PASS). MCID was set at 8 for mHHS, 9 for HOS–Activities of Daily Living (ADL), and 6 for HOS–Sport (SSS). PASS thresholds were set at 74 for mHHS, 87 for HOS-ADL, and 75 for HOS-SSS. Results were considered significant at P <.05 or if the 95% confidence interval for odds ratios did not contain 1.0 (α = 0.05). All statistical analyses were performed using the statistical computing language R version 4.4.0 (R Core Team).

Results

Demographics and Radiographic Parameters

Fifty patients (50 hips) met the inclusion criteria for this study, with 43 (86%) patients reporting a minimum 2-year follow-up. The median age at time of surgery was 45 (range, 19-63) years, and most patients were male (n = 23, 53%). Additionally, 20 (47%) patients had an Outerbridge grade 3 or 4 defect, and 5 (12%) patients underwent a microfracture procedure at the time of surgery. Most patients were considered Tönnis grade 1 (n = 20, 56%), and the mean joint space was 3.00 ± 0.96 mm. Median α angle was 72° (range, 52°-91°), lateral center edge angle was 30° (range, 23°-41°), and median Sharp’s angle was 39° (range, 30°-58°). Notably, most patients had 1 cyst (n = 38, 88%), with 5 (12%) patients having 2 cysts. The median cyst volume was estimated to be 60 mm ³ (range, 4-890 mm ³) when fitting the cyst to an ellipsoid shape and 115 mm ³ (range, 8-1,699 mm ³) when fitting the cyst to a rectangular prism. Additional demographic and radiographic parameters are in Table 1 .

Table 1

Demographic and Preoperative Radiographic Characteristics of Patients With a Complete 2-Year Follow-Up (N = 43)

Characteristic	Value
Follow-up, y (SD)	3.4 (1.4)
Age, median (range)	45 (19-63)
Sex
Male	23 (53)
Female	20 (47)
Outerbridge grade 3 or 4 defect
Yes	20 (47)
No	23 (53)
Impingement pathology addressed
Mixed	42 (98)
Cam	1 (2)
Pincer	0
Microfracture
Yes	5 (12)
No	39 (88)
Tönnis grade
0	15 (42)
1	20 (56)
2	1 (3)
Joint space, mm (SD)	3.00 (0.96)
α angle, ° (range)	72 (52-91)
LCEA, ° (range)	30 (23-41)
Sharp’s angle, ° (range)	39 (30-58)
Number of cysts
1	38 (88)
2	5 (12)
Cyst volume, mm ³ (range)
Ellipsoid	60 (4-890)
Rectangular prism	115 (8-1,699)