Abstract
Background
The risk of developing avascular necrosis (AVN) in the setting of an unstable slipped capital femoral epiphysis (SCFE) that is undergoing treatment with the modified Dunn procedure is not well understood. In addition, since the Loder classification of unstable is reportedly different than actual intraoperatively observed instability (that is, discontinuity between the femoral head epiphysis and proximal femoral metaphysis), the overall risk of developing AVN, as well as the potential complications of treatment of these patients with the modified Dunn procedure, are unknown.
Questions/purposes
To evaluate the modified Dunn procedure for the treatment of patients with epiphyseal-metaphyseal discontinuity, we asked: (1) What was the survivorship free from AVN at 10 years? (2) What was the survivorship free from subsequent surgery and/or complications at 10 years? (3) What were the clinical and patient-reported outcome scores?
Methods
In a retrospective analysis, we identified 159 patients (159 hips) treated with a modified Dunn procedure for SCFE between 1998 and 2020, of whom 97% (155 of 159) had documentation about intraoperatively observed epiphyseal-metaphyseal stability. Of those, 37% (58 of 155) of patients were documented to have intraoperatively observed epiphyseal-metaphyseal discontinuity and were considered eligible for inclusion, whereas 63% (97 of 155) had documented epiphyseal-metaphyseal stability and were excluded. No patients were lost to follow-up before the 2-year minimum. All patients were assessed for survival, but 7% (4 of 58) did not fill out our outcomes score questionnaire. This resulted in 93% (54 of 58) of patients who were available for outcome score assessment. Additionally, 50% (29 of 58) of patients had not been seen within the last 5 years; they are included, but we note that there is uncertainty about their status. The median (range) age at surgery was 13 years (10 to 16), and the sex ratio was 60% (35 of 58) male and 40% (23 of 58) female patients. Sixty-four percent (37 of 58) of patients were classified as acute-on-chronic, and 17% (10 of 58) of patients were classified as acute. Forty-seven percent (27 of 58) of patients presented with severe slips and 43% (25 of 58) of patients with moderate slips based on radiographic classification. All patients underwent surgical hip dislocation with the modified Dunn procedure to correct the slip deformity and provide stabilization. Complications and reoperations were assessed from a review of electronic medical records, and a Kaplan-Meier estimator was used to estimate survivorship free from complications and reoperations at 10 years. Clinical examination results and questionnaire responses were evaluated at minimum 2-year follow-up.
Results
Kaplan-Meier survivorship free from AVN was 93% (95% CI 87% to 100%) at 10 years. Survivorship free from any reoperation was 75% (95% CI 64% to 88%) at 10 years. In addition, survivorship free from complications, defined as development of AVN, reoperation, or a Sink Grade II complication or higher, was 57% (95% CI 45% to 73%) at 10 years. The median (range) Merle D’Aubigne Postel score was 18 (14 to 18) for the patients who did not develop AVN, and 12 (6 to 16) for the four patients who developed AVN (p < 0.001). The median modified Harris hip score was 100 (74 to 100) in the non-AVN cohort and 65 (37 to 82) in the AVN cohort (p = 0.001). Median HOOS total score was 95 (50 to 100) in the non-AVN cohort and 53 (40 to 82) in the AVN cohort (p = 0.002).
Conclusion
Although the modified Dunn procedure is technically challenging, this study shows that in experienced hands, patients with who have demonstrated epiphyseal-metaphyseal discontinuity can be treated with a low risk of AVN and subsequent surgery. Referral of these patients to specialists who have substantial expertise in this procedure is recommended to improve patient outcomes. Prospective, long-term observational studies will help us identify these high-risk patients preoperatively and determine the long-term success of this procedure.
Level of Evidence
Level IV, therapeutic study.
Introduction
The modified Dunn procedure, when performed via a surgical hip dislocation approach, has been reported as a treatment option for slipped capital femoral epiphysis (SCFE), especially in patients with moderate-to-severe slip angles [44]. It not only allows for stabilization of the slip, but also correction of any slip deformity through capital realignment. In the setting of unstable slips (based on the Loder classification), proponents of the modified Dunn procedure draw contrast with in situ pinning by highlighting that this approach allows the femoral head epiphysis to be stabilized and for deformities resulting from the slip to be corrected [3, 22, 23]. Traditional in situ pinning for unstable SCFE, with or without closed reduction, is associated with the development of AVN in 10% to 60% of patients [2]. In addition, using the modified Dunn procedure, Ziebarth et al. [43] demonstrated a poor correlation between the Loder definition of unstable and intraoperatively observed instability demonstrated by visible discontinuity between the epiphysis and the metaphysis (epiphyseal-metaphyseal discontinuity).
When the modified Dunn procedure is performed at centers with substantial experience, the reported risk of AVN for patients with SCFE, regardless of stability classification, is reportedly between 0% and 26.7% [1, 5, 6, 40, 41, 45]. The AVN risk in the setting of an unstable SCFE that is undergoing treatment with the modified Dunn procedure, however, is unknown. In addition, since the Loder classification of unstable is reportedly different than the intraoperatively observed epiphyseal-metaphyseal discontinuity, the overall risk of developing AVN, as well as the potential treatment complications for patients undergoing the modified Dunn procedure, is unknown.
To evaluate the modified Dunn procedure for the treatment of SCFE patients with epiphyseal-metaphyseal discontinuity, we asked the following questions: (1) What was the survivorship free from AVN at 10 years? (2) What was the survivorship free from subsequent surgery and/or complications at 10 years? (3) What were the clinical and patient-reported outcome scores?
Patients and Methods
Study Design and Setting
This was a retrospective case series assessing patients with SCFE and intraoperatively confirmed epiphyseal-metaphyseal discontinuity who were treated at the Inselspital, University of Bern, Switzerland. This is a tertiary-care teaching hospital that specializes in the treatment of young adult hip disease. All patients were operated on by one of four surgeons with subspecialty training in surgical hip dislocation and the modified Dunn procedure.
Patients
From 1998 until approximately 2010, virtually all SCFE patients seen at our hospital, regardless of slip angle, were treated with the modified Dunn procedure. Since approximately 2010, we have primarily treated patients with moderate-to-severe SCFE (slip angle > 30º) with the modified Dunn procedure. Given that all patients with SCFE underwent MRI preoperatively, the presence of substantial posterior neck callus, regardless of slip angle, has been a relative indication for the modified Dunn procedure. Patients with SCFE who did not met our criteria for the modified Dunn procedure were offered an in situ pinning with arthroscopic correction of any cam morphology.
In a retrospective analysis, we identified 159 patients (159 hips) treated with a modified Dunn procedure for SCFE between 1998 and 2020, of whom 97% (155 of 159) had documentation about intraoperatively observed epiphyseal-metaphyseal stability. Of those, 37% (58 of 155) were documented to have intraoperatively observed epiphyseal-metaphyseal discontinuity and were considered eligible for inclusion, and 63% (97 of 155) had documented epiphyseal-metaphyseal stability and were excluded. Though not included in this analysis, a review of these 97 patients shows that none had developed AVN by most recent follow-up. All 58 patients were assessed for survival, but not all completed outcomes scores or got radiographs. No patients were lost to follow-up before the 2-year minimum; however, 3% (2 of 58) of patients were not included in the outcomes score assessment as they live abroad and were unreachable for follow-up, and an additional 3% (2 of 58) of patients were unable to complete the outcomes scores because of cognitive abilities. This resulted in 93% (54 of 58) of patients who were available for outcome score assessment (Fig. 1). Additionally, 50% (29 of 58) of patients had not been seen within the last 5 years; they are included, but we note that there is uncertainty about their status. Median (range) follow-up was 10 years (3 to 17) for those who have not been seen in the last 5 years and 11 years (2 to 22) for those who have been seen in the last 5 years (p = 0.70).
Fig. 1.
Flowchart shows the exclusion criteria of patients with unstable SCFE (epiphyseal-metaphyseal discontinuity) for this study.
The clinical follow-up with questionnaires was carried out specifically for this study with clinical visits where patients were called back in at a mean follow-up of 10 (2 to 23) years after treatment.
Descriptive Data
The median (range) age at surgery was 12.7 years (9.8 to 15.8) (Table 1). The sex ratio was 60% (35 of 58) male and 40% (23 of 58) female patients, and 71% (41 of 58) involved the left hip (Table 1).
Table 1.
Demographic and clinical data of the patients with unstable SCFE (epiphyseal-metaphyseal discontinuity)
| Parameter | No AVN (n = 54) | AVN (n = 4) | p value |
| Age at operation in years | 13.0 (9.8-15.8) | 11.9 (10.7-14.6) | 0.6 |
| Age at latest follow-up in years | 22.2 (12.9-35.9) | 19.0 (16.4-21.1) | 0.063 |
| Sex | > 0.9 | ||
| Female | 41 (22) | 25 (1) | |
| Male | 59 (32) | 75 (3) | |
| Side | 0.6 | ||
| Right | 28 (15) | 50 (2) | |
| Left | 72 (39) | 50 (2) | |
| Height in cm | 160 (145-180) | 150 (135-165) | 0.5 |
| Weight in kg | 60 (30-95) | 50 (47-69) | 0.8 |
| BMI in kg/m2 | 22.2 (15.8-34.4) | 27.4 (27.4-27.4) | 0.5 |
| Follow-up time in years | 10.3 (2-23) | 7.1 (2-10.3) | 0.13 |
| Duration of symptoms in weeks | 4 (0-34) | 3 (2-8) | 0.7 |
| Loder classification | > 0.9 | ||
| Stable | 70 (38) | 75 (3) | |
| Unstable | 30 (16) | 25 (1) | |
| Unstable hips based on intraoperative epiphyseal-metaphyseal discontinuity [43] | 100 (54) | 100 (4) | |
| Severity based on slip angle [35] | > 0.9 | ||
| Mild < 30° | 11 (6) | 0 (0) | |
| Moderate 30°-60° | 43 (23) | 50 (2) | |
| Severe > 60° | 46 (25) | 50 (2) | |
| Classification based on the duration of symptoms [2, 29]a | 0.8 | ||
| Chronic | 20 (11) | 0 (0) | |
| Acute-on-chronic | 63 (34) | 75 (3) | |
| Acute | 17 (9) | 25 (1) |
Data presented as median (range) or % (n).
Acute is defined as symptoms less than 3 weeks, chronic is defined as symptoms longer than 3 weeks and a gradual onset, acute-on-chronic is defined as symptoms longer than 3 weeks with a sudden acute exacerbation lasting less than 3 weeks.
Preoperative radiographs consisted of AP and cross-table lateral or Lauenstein view radiographs. Median preoperative slip angle was 50° (15° to 80°). Forty-seven percent (27 of 58) of patients presented with severe slips and 43% (25 of 58) of patients with moderate slips based on radiographic classification (Table 1). Sixty-four percent (37 of 58) of patients had symptom duration classified as acute-on-chronic, and 17% (10 of 58) of patients had hips that were classified as acute (Table 1). All patients underwent surgical hip dislocation with the modified Dunn procedure to correct the slip deformity and provide stabilization.
Surgical Technique and Rehabilitation
For the modified Dunn procedure, the patient is placed in the lateral decubitus position. It is mandatory that a member of the operative team directly supervises patient positioning to prevent unintentional rotation or traction of the affected leg, which would compromise the femoral head blood supply. The detailed surgical technique of the modified Dunn procedure has been reported before [45]. After capsulotomy, femoral head stability is checked by gentle flexion and external rotation of the leg. In patients with an observed epiphyseal-metaphyseal discontinuity, before attempting surgical dislocation, the surgeon applies a temporary fixation to the femoral head by inserting two K-wires through the anterior aspect of the neck into the head to prevent unintentional complete slipping of the epiphysis from the neck during further manipulation.
In severe slips (Southwick angle > 60°) [11] and those with markedly unstable epiphyseal-metaphyseal junctions, alteration or laceration of the retinacular vessels may have already taken place during the slip [23, 33] (Fig. 2). Therefore, it is mandatory to evaluate the femoral head blood supply before and after the capital realignment procedure, especially in hips with visible epiphyseal-metaphyseal discontinuity. This is accomplished by drilling a hole in the femoral head with a small (1.5 mm to 2.0 mm) drill bit. Active bleeding from the hole confirms blood supply preservation. Alternatively, a laser Doppler probe may be inserted in the femoral head. A simultaneous wave between the heartbeat (EKG wave) of the patient and the pressure in the femoral head on the monitor confirms femoral head perfusion.
Fig. 2.
(A) A 12-year-old female patient with SCFE was unable to walk and had an unstable SCFE (Loder classification) and a severe SCFE (slip angle > 60°). She was treated with a modified Dunn procedure and intraoperative findings were: (B) periosteum ruptured (black star), retinacular vessels/retinaculum (tip of forceps) intact, (C) no bleeding from drill hole (black arrow), and (D) bleeding from drill hole after release of retinacular vessels by retinacular flap. (E-F) Radiographs 2 years postoperatively show intact and round femoral head and anatomical head-neck offset. The patient had no hip pain and free hip ROM.
After fixation of the unstable head with K-wires to the neck, the proximal femur can be dislocated from the socket without harming the retinaculum. This provides full visualization of the acetabulum and allows cartilage assessment. If labral tears or cartilage flaps are observed, appropriate treatment can be immediately executed before proceeding with the capital realignment procedure.
To reduce the femoral head on the femoral neck, a periosteal/retinacular flap is developed as described in the evidence [7, 20, 21, 45]. The superior periosteal retinaculum containing the terminal branches of the blood supply to the epiphysis is carefully protected. Once the retinacular flap has been prepared, the K-wires are carefully withdrawn. The femoral head is placed in the acetabulum with rigorous attention to the integrity of the retinacular flap.
The residual femoral neck and metaphysis is now inspected for posterioinferior callous formation, which in our experience is present in most hips in this setting. Callous resection is required to allow anatomical reduction of the femoral head without excessive traction or compression of the retinacular vessels within the periosteal flap. Typically, the proximal stump of the femoral neck is shortened approximately 5 to 7 mm and cut perpendicular to the base of the neck. The trimming is performed circumferentially to allow adequate fit and position of the epiphysis. Finally, to ensure bone-to-bone healing, the remaining white-appearing physis at the base of the femoral head must be completely removed using a curette or a drill before reduction.
Once the epiphyseal and metaphyseal fragments are prepared, the femoral head is reduced anatomically on the femoral neck with constant attention to the integrity and tension of the retinacular flap. Positioning the epiphysis with adequate head-neck offset in the anterior and superior quadrants is crucial.
Postoperatively, the patient is permitted to do touchdown weightbearing with the use of crutches for 6 weeks after surgery. The patient’s weightbearing is gradually increased to full after radiographic confirmation of greater trochanteric osteotomy healing. At 3 months postoperatively, the patient’s rehabilitation status is evaluated based on their ROM and abductor function.
In our series, we observed that 7% (4 of 58) of patients had compromised blood supply (that is, no bleeding after drilling of the epiphysis and no signal with the laser probe). For these patients, partial weightbearing on crutches was maintained for 3 months.
Outcomes Scores
The Merle d’Aubigné-Postel score was derived for all patients from the most recent clinical follow-up visit for their treated hip. In addition, as noted, 50 patients completed a standardized questionnaire sent via post and/or email, based on patient’s preference, including Harris hip score [8] and the Hip Disability and Osteoarthritis Outcome Score (HOOS) [14, 28].
The maximum Harris hip score is 100 points; a score of 91 to 100 points corresponded to excellent hip function, 81 to 90 points signaled good function, 71 to 80 points was categorized as fair function, and ≤ 70 points was rated as poor hip function. The HOOS consists of 40 items assessing five dimensions: pain, function in activities of daily living (ADL), function in sport and recreation, hip-related quality of life (QOL), and symptoms and stiffness. Each subscale has a score and is reported from 0 to 100% of that score, where 0 indicates extreme problem and 100% indicates no problem. Given that we did not have preoperative scores on these patients, we are presenting results from their most-recent follow-up. Typically in reconstructive series, it is important to compare preoperative and postoperative scores; however, in this situation, it is fair to assume that the premorbid condition of these patients—that is, their scores before experiencing SCFE—was normal or nearly so.
Clinical Examination
In addition to assessing healing, physical examinations during the follow-up period were also performed to evaluate for hip ROM, femoroacetabular impingement (FAI) using the anterior impingement (flexion-adduction-internal rotation or FADIR) test [37], limited internal rotation of the hip at 90° flexion, and abnormal femoral head-neck morphology on imaging.
Data on patients who underwent any additional or subsequent surgical treatment during the study period were recorded from the medical records (Table 2). Amplitudes of hip motion in flexion, abduction, adduction, as well as internal and external rotation assessed with the hip in 90° of flexion were also recorded. Different observers performed the clinical evaluations preoperatively and at follow-up. However, substantial inter- and intraobserver agreement has been reported for the anterior impingement test [26], ROM [9, 27, 42], the Merle d’Aubigné-Postel score [13], as well as the Drehmann sign in SCFE patients [12].
Table 2.
Subsequent surgery and complications in patients who underwent the modified Dunn procedure for SCFE with epiphyseal-metaphyseal discontinuity
| Patients (n = 58) | Sink grade | |
| Subsequent surgeries | 24% (14) | |
| Intertrochanteric varus osteotomy and triple osteotomya | 3% (2) | III |
| Intertrochanteric varus osteotomy and hip arthrodesisa | 2% (1) | III |
| THAa | 2% (1) | III |
| Revision osteosynthesis due to screw breakage | 3% (2) | III |
| HO resection and cam correction | 2% (1) | III |
| Surgical hip dislocation with cam correction and labrum fixation | 2% (1) | III |
| Implant removal | 10% (6) | III |
| Complications treated without subsequent surgery | 17% (10) | |
| HO without surgical treatment (Brooker 1) | 12% (7) | II |
| Broken K-wires without surgical treatment | 5% (3) | II |
Data presented as % (n). Sink Grade II was defined as deviation from normal postoperative course resulting in outpatient treatment or close monitoring, and Sink Grade III was defined as a complication resulting in further surgery or unplanned hospital admission [34].
Treatment for patient who developed avascular necrosis; HO = heterotopic ossification.
Complications were graded according to the Dindo-Clavien classification [4], which was adapted for orthopaedic surgery by Sink et al. [34]. Grade I complications undergo no treatment, have no clinical relevance, and have no deviation from routine follow-up. Grade II complications undergo deviation from the normal postoperative course and outpatient treatment. Grade III complications undergo surgical or radiological interventions or an unplanned hospital admission. Grade IV complications are life threatening or have the potential for permanent disability. Death is a Grade V complication [4, 34].
Radiographic Examination
Radiographic evaluation included an AP pelvis radiograph and cross-table lateral or Lauenstein view. Preoperative AP radiographs were evaluated by one of the authors (TDL) not involved in the clinical care of the patients. Slippage of the epiphysis was quantified using the slip angle [36] and the intactness of Klein’s line [15].
Postoperative radiographic assessment included the slip angle, the alpha angle [29], and joint space (less than 3 mm was considered chondrolysis). Asphericity and femoral head subchondral fracture were considered as signs of AVN.
Primary and Secondary Study Outcomes
Our primary study goal was to determine the 10-year AVN-free survivorship of patients with epiphyseal-metaphyseal discontinuity undergoing treatment with the modified Dunn procedure. To achieve this, we performed a Kaplan-Meier survival analysis on our cohort using AVN as the primary endpoint.
Our secondary study goals were to determine the 10-year survivorship free from subsequent surgery or major complications and report on the clinical, functional, and radiographic outcomes of patients with epiphyseal-metaphyseal discontinuity undergoing treatment with the modified Dunn procedure. To achieve these goals, we performed a Kaplan-Meier survival analysis on our cohort using subsequent surgery and major complications (Sink Grade II or higher) as a primary endpoint, and we collected the Merle d’Aubigné-Postel score, modified Harris hip score, HOOS, hip ROM, Drehmann sign, and presence of a positive anterior impingement test.
Ethical Approval
We obtained institutional review board approval for this study from Kantonale Ethikkommission Bern (ID 2018-00078).
Statistical Analysis
Statistical analysis was carried out with Winstat software (R. Fitch Software) and R/RStudio (The R Foundation for Statistical Computing). Pre- and postoperative continuous parameters were compared with unpaired double-tailed Student t-tests. The level of significance was p < 0.05. Kaplan-Meier survivorship at 10-year follow-up was calculated separately for three different endpoints: AVN, subsequent surgery, and major complication (defined as development of AVN, reoperation, or a Sink Grade II complication or higher).
Results
10-year AVN-free Survivorship
Kaplan-Meier survivorship free from AVN was 93% (95% CI 87% to 100%) at 10 years (Fig. 3A). In addition, with the numbers available, no difference in 10-year AVN-free survivorship was detected between SCFEs classified preoperatively as Loder stable (93% [95% CI 85% to 100%]) and those classified as Loder unstable (94% [95% CI 84% to 100%; p = 0.90]).
Fig. 3.
Kaplan-Meier 10-year survival estimators with endpoints of (A) avascular necrosis (AVN), (B) subsequent surgery, and (C) major complication (defined as development of AVN, reoperation, or a Sink Grade II complication or higher).
10-year Survivorship Free From Subsequent Operations and Complications
Survivorship free from any reoperation was 75% (95% CI 64% to 88%) at 10 years (Fig. 3B). In addition, survivorship free from complications, defined as development of AVN, reoperation, or a Sink Grade II complication or higher, was 57% (95% CI 45% to 73%) at 10 years (Fig. 3C). The most common reason for reoperation was implant removal, which occurred in 10% (6 of 58) of patients, and the most common complication overall, occurring in 12% (7 of 58) of patients, was Brooker 1 heterotopic ossification treated without surgery (Table 2).
Clinical Outcomes
The median (range) Merle D’Aubigne-Postel score was 18 (14 to 18) for the patients who did not develop AVN and 12 (6 to 16) for the four patients who developed AVN (p < 0.001) (Table 3). The median (range) modified Harris hip score was 100 (74 to 100) in the non-AVN cohort and 65 (37 to 82) in the AVN cohort (p = 0.001). The median (range) HOOS total score was 95 (50 to 100) in the non-AVN cohort and 53 (40 to 82) in the AVN cohort (p = 0.002). Median flexion was 110º (80º to 130º) in patients without AVN and 80º (70º to 90º) in patients who developed AVN (p = 0.02). At follow-up, 19% (11 of 58) of patients had a positive anterior impingement test.
Table 3.
Clinical outcomes of the patient series with unstable SCFE (epiphyseal-metaphyseal discontinuity) are shown
| Patient-reported outcomes | No AVN (n = 54) | AVN (n = 4) | p value |
| Merle d’Aubigne-Postel Score | 18 (14-18) | 12 (6-16) | < 0.001 |
| Modified Harris hip score [10] | 100 (74-100) | 65 (37-82) | 0.001 |
| Excellent | 81 (44) | 0 (0) | |
| Good | 2 (1) | 25 (1) | |
| Fair | 7 (4) | 25 (1) | |
| Poor | 0 (0) | 50 (2) | |
| HOOS total score [16, 26] | 95 (50-100) | 53 (40-82) | 0.002 |
| Pain | 100 (48-100) | 56 (36-91) | 0.001 |
| Function and daily living | 100 (68-100) | 61 (46-90) | < 0.001 |
| Sports and recreational activities | 100 (50-100) | 49 (19-69) | < 0.001 |
| Symptoms and stiffness | 95 (50-100) | 55 (35-80) | 0.003 |
| Quality of life | 94 (31-100) | 49 (25-81) | 0.004 |
| Hip ROM at follow-up | |||
| Flexion in ° | 110 (80-130) | 80 (70-90) | 0.02 |
| Internal rotation in 90° of flexion in ° | 40 (5-70) | 20 (0-40) | 0.30 |
| External rotation in 90° of flexion in ° | 45 (20-80) | 25 (0-50) | 0.40 |
| Abduction in neutral position in ° | 40 (30-50) | N/A | |
| Adduction in neutral position in ° | 20 (10-30) | N/A | |
| Positive Drehmann sign [7, 14]a | 0 (0) | 25 (1) | 0.02 |
| Positive anterior impingement test % (n) | 19 (10) | 25 (1) | 0.40 |
Data presented as median (range) or % (n). Score rangers were: Merle d’Aubigne-Postel score = 0 (worst condition) to 18 (best condition), modified Harris Hip Score = 0 (worst functional outcome and maximum pain) to 100 points (best functional outcome and least pain), HOOS total score = 0 points (worst possible score) to 100 points (best possible score), HOOS pain = 0 to 40 points, HOOS function and daily living = 0 to 68 points, HOOS sports and recreational activities = 0 to 16 points, HOOS symptoms and stiffness = 0 to 20 points, and the HOOS quality of life = 0 to 16 points; all HOOS subscores are presented as % of total possible points per subsection. The positive anterior impingement test is pain with flexion, adduction, and internal rotation of the affected hip.
Drehmann sign is obligatory external rotation of the hip joint during flexion; HOOS = Hip Disability and Osteoarthritis Outcome Scores.
Discussion
Although femoral head AVN is a potential complication of SCFE, patients with Loder unstable SCFE are at a higher risk of AVN. Although this has not yet been shown, it seems reasonable to believe that the risk of developing AVN would be highest when there is a discontinuity between the femoral head epiphysis and metaphysis. Since the Loder classification of stable and unstable does not correlate with intraoperative epiphyseal-metaphyseal discontinuity, we hoped our study would provide important insight into the risk of developing AVN in this high-risk population. We found that the AVN risk after the modified Dunn procedure in this population was low compared with other studies of comparable patients. In addition, the 10-year survivorship free of subsequent surgeries and complications after the modified Dunn procedure in a population with epiphyseal-metaphyseal discontinuity is comparable to other studies on the treatment of Loder unstable slips with in situ pinning [31]. Although other, more minimally invasive procedures such as the Parsch miniopen procedure have reported good results, there are no comparable data about hips demonstrating epiphyseal-metaphyseal discontinuity. In addition, posteroinferior callus cannot be addressed through these approaches, and in an acute-on-chronic SCFE, this could pose a tremendous risk. Finally, the outcomes we report indicate that it is possible to achieve consistently high postoperative functioning and near-normal hip biomechanics with good patient satisfaction when performing this technically challenging procedure in patients at high risk for AVN or premature arthritis.
Limitations
This study has limitations. First, as our experience with the modified Dunn procedure evolved over the 22 years of this study, so did our indications. From 1998 to approximately 2010, nearly all patients with SCFE were treated with a modified Dunn procedure. Starting in approximately 2010, patients with mild SCFE (< 30º) were offered in situ pinning with hip arthroscopy to correct any cam morphology unless they had a preoperative MRI that showed visible posterior neck callus. Preoperative MRI for patients with SCFE was introduced in approximately 2010 and has been performed routinely since 2014 [17, 18]. Given that this study intended to investigate the outcome of the modified Dunn procedure in the setting of epiphyseal-metaphyseal discontinuity, the available published evidence could be used to compare our AVN rates with published studies about the risk of this complication.
Another limitation is that complete data for clinical and radiographic follow-up were only available for all patients for the first 2 years. Although mean 10-year follow-up data are available for a large portion of our cohort, we feel that AVN because of either the SCFE itself or the surgical treatment for the SCFE should be apparent by 2 years. The fact that no AVN was noted after 6 months in our cohort strengthens this supposition. This relatively small number of patients with AVN also limits our ability to identify relevant risk factors for the development of AVN or other complications. In addition, different observers performed the clinical examination preoperatively and at follow-up. This is because of the retrospective design and extended follow-up period of more than 15 years in some patients (1998 to 2020). This may have introduced bias into our results. However, substantial inter- and intraobserver agreement has been reported for the Merle d’Aubigné-Postel score [13], the anterior impingement test [26], the Drehmann sign in patients with SCFE [12], and hip ROM [9, 27, 42].
We note that epiphyseal-metaphyseal discontinuity in our patients was defined intraoperatively, clearly demonstrating a mechanically unstable SCFE. By contrast, most other studies, especially those dealing with in situ pinning, based their judgement of stability symptoms according to the clinical findings recommended by Aronsson et al. [3] and Loder et al. [24]. This may lead to different study groups because the clinical estimation of slip stability poorly correlates with intraoperative findings [43]. Finally, since this series spans over two decades, the surgical technique itself, indications for this procedure, and individual surgeon experience has substantially evolved over that time span. The surgeons involved in this study are highly specialized in this technique, however, and we believe that our results justify the use of this treatment and provide tremendous potential for drastically improving the outcomes in this population that is at high risk of poor outcomes with conventional treatment.
10-year AVN-free Survivorship
At 10-years’ follow-up, we found that nearly all patients in our study were free from AVN. We find this outcome to be promising; prior studies have suggested that AVN occurs in 28% to 58% of hips when treating patients who have Loder unstable SCFE with in situ pinning or casting [2, 30, 38]. The modified Dunn procedure is technically challenging, and AVN has occurred in other studies in 14% to 29% of patients who have undergone this procedure [25, 30, 32, 35]. We feel that with adequate volume and training in the modified Dunn procedure, we can reduce the AVN risk in these patients.
10-year Survivorship Free From Subsequent Operations and Complications
In our study, the risk of reoperations and major complications was low after treatment of SCFE with epiphyseal-metaphyseal discontinuity treated with the modified Dunn procedure. The subsequent surgeries and complications in our series were comparable to other studies investigating the modified Dunn procedure [16, 19, 44]. The risk for implant breakage or joint penetration, which ranges from 7% to 15%, may be substantially related to the fixation type used [10, 30, 32]. In our experience, there seems to be an increased risk of implant breakage when smaller-diameter cortical screws are used compared with threaded wires [10, 32]. Thus, we now primarily use fully-threaded, 3.5-mm wires in our institution. In addition, the low number of reoperations in our series for abnormal postoperative hip biomechanics and/or FAI shows the positive impact of this procedure when AVN is avoided.
Clinical Outcomes
We observed near-normal hip function and minimal hip pain with good patient-related outcome scores in patients at a mean 10-year follow-up, other than in patients who developed AVN, whose clinical scores (not surprisingly) were much poorer. This study highlights that one major benefit of the modified Dunn procedure over other treatment options is the improvement in long-term hip function and biomechanics. Given that this procedure not only provides stability in the setting of SCFE, allowing fusion across the affected physis, but also corrects the abnormal mechanical relationship between the femoral head and femoral neck, the risk of poor hip biomechanical functioning because of malalignment or post-SCFE impingement is substantially reduced. The outcomes we observed are comparable to previous studies reporting clinical outcome scores for the modified Dunn procedure for patients with mild, moderate, and severe SCFE, independent of physeal stability [10, 30, 44].
Possible Etiologies of AVN in This Series
Two of four patients who developed AVN in our study demonstrated no visual bleeding from the femoral head after intraoperative drilling with a 2.0-mm drill before any attempt at reduction. In both instances, the superior retinaculum was observed to be ruptured or lacerated at initial visualization (Fig. 4). One patient experienced a 3-week delay between the initial onset of sudden hip pain during gymnastic exercises (maximum flexion and extension) and the diagnosis of a severe and unstable slip. Despite this delay, we felt the severity and stability of the slip would best be treated with a modified Dunn procedure. The fourth patient showed sparse bleeding of the femoral head from a drill hole after capsulotomy before any attempt for reduction; however, after capital realignment, no more bleeding from the drill hole was observed. The reason for this is unknown; however, with no documented cause, surgical error cannot be ruled out. As a disease process, the occurrence and incidence of AVN is not clearly understood and may have a number of different etiologies. In the situation of an SCFE with epiphyseal-metaphyseal discontinuity, it seems obvious that the superior retinaculum is at highest risk for being lacerated, as this would represent the highest grade of instability in slips. Surgery that is delayed by days or weeks, as in our third patient, may lead to kinking or thrombotic occlusion of the terminal branches of the femoral head blood supply. This would be difficult for the surgeon to detect during treatment. In addition to these factors, differences in the execution of the surgical technique (closed reduction, in situ pinning, open reduction) and the skills of the respective surgeons [32] must be considered when analyzing adverse events, especially AVN of the epiphysis. Upasani et al. [39] recommended the attendance of a high-volume hip surgeon when performing the modified Dunn procedure.
Fig. 4.
(A-B) A 14-year-old male patient with SCFE who was able to walk. The line of Klein runs through the epiphysis (white line). (C) The patient was treated with a modified Dunn procedure, and intraoperative findings after capsulotomy showed a complete rupture of the retinacular vessels (black star). (D) A radiograph obtained 3 months postoperatively shows visible asphericity of the femoral head. (E) Radiographs after partial hardware removal and (F) 6 months postoperatively show progressive AVN and destruction of the femoral head. (G) The patient underwent total hip replacement 1 year postoperatively.
Conclusion
Although the modified Dunn procedure is technically challenging, this study shows that SCFE hips with demonstrated epiphyseal-metaphyseal discontinuity can be treated with a low risk of AVN and subsequent surgery. Given that with other approaches these patients have been at very high risk for severe complications, in particular AVN, referral of these patients to specialists who have substantial expertise in this procedure is recommended. Prospective, long-term observational studies are necessary to help identify true epiphyseal-metaphyseal discontinuity preoperatively and to assess the longevity of joint preservation, preventing or delaying the development of osteoarthritis, using this surgical technique.
Footnotes
The first two authors contributed equally to this manuscript.
Each author certifies that there are no funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article related to the author or any immediate family members.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.
Ethical approval for this study was obtained from Kantonale Ethikkommission Bern (ID 2018-00078).
The work was performed at the Inselspital Bern, University of Bern, Bern, Switzerland.
Contributor Information
Kai Ziebarth, Email: kai.ziebarth@insel.ch.
Tilman Kaim, Email: tilman.kaim@insel.ch.
Joseph M. Schwab, Email: joseph.schwab@unifr.ch.
Simon D. Steppacher, Email: simondamian.steppacher@insel.ch.
Moritz Tannast, Email: moritz.tannast@unifr.ch.
Klaus A. Siebenrock, Email: klaus.siebenrock@insel.ch.
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