Surgical treatment of proximal femur unicameral bone cysts in children: a retrospective single-center study

Abstract

Objective

The aim of this study was to present clinical and radiological outcomes of surgical treatment for proximal femur unicameral bone cysts (UBCs) in children.

Methods

Pediatric patients recruited from our institution between 2015 and 2024 with proximal femur UBC were analyzed retrospectively. Patients were divided into two groups according to whether internal fixation was used. Demographics, cyst activity, cyst area, healing time of pathological fractures and cysts, clinical and radiological outcomes, time to activity and complications were analyzed.

Results

Thirty-seven pediatric patients were included in this study. There were 18 patients in the non-internal fixation group and 19 patients in the internal fixation. There was no significant difference between these two groups in terms of age, duration of follow-up, fracture at diagnosis, cyst activity or healing time of pathological fractures and cysts. Cyst area in the internal fixation group was 1609±1131 mm², which was significantly greater than that in the group without internal fixation (936±597 mm², p<0.05). There was no significant difference in clinical or radiological outcomes between the two groups. Patients who were treated with internal fixation were able to return to activity significantly sooner than those who were not (5.9 months vs. 7.7 months; p<0.05). There was no significant difference in complications between the two groups (p=1.000).

Conclusions

Surgical treatment of proximal femoral UBCs in children remains a challenge. The use of internal fixation is beneficial for accelerating the healing process and reducing the time to activity.

WHAT IS ALREADY KNOWN ON THIS TOPIC

• The management of proximal femur unicameral bone cysts (UBC) in children presents a challenge due to the weight-bearing nature of the anatomy, the potential for recurrence and the intricacies associated with securing stable fixation.

WHAT THIS STUDY ADDS

• In children diagnosed with UBC of the proximal femur, those in the non-internal fixation group who underwent curettage and bone grafting for relatively smaller cysts exhibited comparable outcomes to those in the internal fixation group who underwent the same procedures for larger cysts. A relatively high recurrence rate is a major concern. Utilizing internal fixation is beneficial for accelerating the healing process and reducing the time to activity.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

• Curettage and bone grafting with internal fixation might gradually become a preferred treatment for proximal femur UBC in children.

Introduction

Unicameral bone cysts (UBCs) are benign bone tumors and tumor-like lesions characterized by a cystic cavity; they account for approximately 3% of all benign bone tumors.¹ UBC usually start in the metaphysis or meta-diaphysis of long bones, especially the proximal humerus and proximal femur and affects mainly children and adolescents with immature skeletons. The typical histological manifestations include cavities filled with interstitial fluid and thin fibrous membranes on the inner surface. They are rich in prostaglandins, giant cells, histiocytes and chronic inflammatory cells.² With respect to etiology, several hypotheses have been advanced, but this remains poorly understood.³ One of the most widely accepted hypotheses is obstruction from the intramedullary vein, leading to accumulation of exudate.⁴

Owing to its unique anatomical structure, the proximal femur is an important weight-bearing region in the lower extremity. Unlike other non-weight-bearing regions, UBCs located at the proximal femur are more prone to pathological fracture; thus, more aggressive surgery is needed.⁵ Prevention and management of pathological fractures, achieving cyst consolidation and preventing refracture and recurrence are the ultimate therapeutic goals. To date, various surgical procedures, such as intralesional injection of methylprednisolone, decompression, curettage, grafting and internal fixation, have been reported. There is still no consensus on the optimal therapeutic approach.^{6 7}

Curettage and bone grafting is a classical treatment alternative for UBC. Although a combination with internal fixation is preferred, treatment of proximal femur UBC is suboptimal because of the relatively high failure and complication rates.⁸ In our retrospective study, the primary objective was to present both clinical and radiological outcomes of proximal femur UBC in children treated with curettage and bone grafting with or without internal fixation. Our secondary objective was to evaluate the effect of internal fixation on outcomes of proximal femur UBC in children.

Methods

With ethical committee approval and informed consent, patients with UBC of the proximal femur were analyzed retrospectively over the period 2015–2024. All patients were treated with curettage and bone grafting with or without internal fixation (elastic stable intramedullary nails or proximal femoral plates (Xiamen double medical device). Patients with complete clinical and imaging data, patients aged less than 16 years and patients whose primary UBC was located at the proximal femur and confirmed by pathological diagnosis were included in the study. Patients with metabolic bone disease or syndromic disorders such as osteogenesis imperfecta, rickets or Ollier disease were excluded from the study. Patients who underwent operations elsewhere or had less than 6 months of follow-up were also excluded. Thirty-seven patients fulfilled the inclusion criteria and were divided into two groups according to whether internal fixation was used.

The following data were collected: age at diagnosis; sex; clinical presentation; radiological findings; surgical procedure; clinical and radiological outcomes; time to activity; duration of follow-up; complications, including malunion with coxa vara, leg length discrepancy (>1 cm), avascular necrosis and refracture; pathological fracture, active (<5 mm from the physis) or latent; and size of the cyst, assessed by plain radiograph taken just before surgery. The area of the cyst was measured by drawing one or more trapezoids and using the equation of (long base+short base)/2×in mm².

Surgical technique

All surgical procedures were performed by experienced senior surgeons. Patients were placed in a supine position, elevating the hip to approximately 30°. After general anesthesia, a Watson‒Jones approach was used to expose the proximal femur. If the tumor was located at the femoral neck, a Smith‒Petersen approach was used to increase exposure of the lesion. In cases without internal fixation, a suitable curette was used to resect of the tumor through a lateral cortical window or fracture.

No burring or adjuvant was used. Tumor samples obtained during the process were sent for pathological examination. Gross reduction of the bony fragments was performed when the patient experienced a displaced fracture. After the defect was filled with beta tricalcium phosphate (Shanghai Rebone Biomedicals) and/or autogenous iliac bone graft, a hip spica was applied. In cases with internal fixation, two intramedullary nails or proximal femoral plates were utilized to maintain reduction or stability. A hip spica or brace was applied. When needed, image intensifier fluoroscopy was employed to complete all procedures.

Postoperative patient management

A hip spica or brace was applied to every patient for 6–8 weeks. A rehabilitation program was developed to increase mobility of hip joint and strengthen the operated extremity. When the spica or brace was removed, patients were allowed crutch-assisted walking without bearing weight. When there was obvious opacification with cortical thickening or osteotylus in radiological follow-up, patients were permitted partial weight bearing. Patients were gradually permitted to progress from partial weight bearing to full weight bearing. The time to activity was recorded.

Clinical and radiological follow-up

The follow-up period was 2 to 4 weeks until fracture union or cyst consolidation, 3 monthly in the following year and 6 monthly thereafter. Follow-up continued until skeletal maturity was reached. Plain radiography was conducted at each follow-up. Healing time of pathological fractures and cysts was determined on the basis of serial plain radiography. Healing time of pathological fracture was defined as the postoperative period when the fracture line blurred or disappeared. Cyst healing time was defined as when there was complete or incomplete opacification at regular follow-up.⁹ Radiologic outcomes were evaluated using the Capanna classification, which was categorized as healed, healed with residual, recurrent and no response.¹⁰ Musculoskeletal Tumor Society (MSTS) scores for the lower limb, which are categorized as excellent, good, fair or poor, were used to evaluate clinical outcomes.¹¹ Complications, including malunion with coxa vara, leg length discrepancy (>1 cm), avascular necrosis and refracture, were analyzed via radiography at the last follow-up.

Statistical analysis

Quantitative data (age at diagnosis, duration of follow-up, cyst area, healing time of pathological fracture and cyst and time to activity) are presented as mean and standard deviation (SD). Qualitative data (gender, cyst activity, fracture at diagnosis, MSTS score, Capanna classification and complication rate) are presented as numbers and percentages. Statistical analysis was performed using SPSS V.19.0. Differences in quantitative data were compared using independent-sample t tests. Differences in qualitative data were compared using Fisher’s exact tests. A single asterisk (*) is defined as a statistically significant difference at p<0.05.

Results

Totally, 37 patients with a mean age of 7.4±3.0 years (range, 2.3–13.1 years) were retrospectively included and were followed for a meanduration of 4.5±2.3 years (range, 2.0–9.1 years) . There were 18 patients in non-internal fixation group (figure 1) and 19 in internal fixation group (figure 2). Of the internal fixation cases, 15 used elastic stable intramedullary nails and 4 used proximal femoral plates. Thirty (81.1%) of the patients were male, and seven (18.9%) were female. Eight (21.6%) patients had active cysts and 29 (78.4%) patients had latent cysts. Thirteen (35.1%) patients presented with pathological fractures. Healing time of pathological fractures was 2.4±0.5 months (range, 1.5–3.0 months) and of cysts was 5.6±1.8 months (range, 3.0–10.0 months). Demographic characteristics of the two groups are shown in table 1. Gender, age, follow-up duration, fracture at diagnosis, cyst activity, healing time of pathological fractures and cysts did not differ between two groups. Cyst area of all 37 patients was 1281±962 mm² (range 156–4576 mm²). Cyst area in the internal fixation group was 1609±1131 mm², which was significantly greater than that in the other group (936±597 mm², p<0.05).

Figure 1. Radiograph of a patient with unicameral bone cyst of the proximal femur. The patient was treated with curettage and bone grafting without internal fixation. (A) Preoperative plain radiograph. (B) Postoperative radiograph at 1 day postsurgery. (C) Postoperative radiograph at 6 years.

Figure 2. Imaging of a patient with unicameral bone cyst of the proximal femur. The patient was treated with curettage and bone grafting with a proximal femoral plate. (A-C) Preoperative plain radiograph, CT and MRI. (D) Postoperative radiograph at 1 day post surgery. (E-F) Postoperative radiograph at 3 years.

Table 1. Patient characteristics and outcomes in the two groups.

Characteristics	Non-internal fixation group (n=18)	Internal fixation group (n=19)	P
Male	14 (77.8)	16 (84.2)	0.693
Age at diagnosis, years	6.5±3.1	7.6±2.6	0.295
Follow-up, years	4.8±2.3	4.3±2.4	0.534
Fracture at diagnosis	6 (33.3)	7 (36.8)	1.000
Active cyst (<5 mm from the physis)	4 (22.2)	4 (21.1)	1.000
Cyst area, mm2	936±597	1609±1131	0.031*
Healing time of fracture, months	2.2±0.5	2.6±0.4	0.159
Healing time of cyst, months	5.5±1.9	5.7±1.8	0.863
MSTS			0.896†
Excellent	10 (55.5)	8 (42.1)
Good	3 (16.7)	4 (21.1)
Fair	2 (11.1)	4 (21.1)
Poor	3 (16.7)	3 (15.7)
Capanna classification			0.657†
Healed	10 (55.5)	7 (36.8)
Healed with residual	3 (16.7)	5 (26.3)
Recurrence	5 (27.8)	6 (31.6)
No response	0 (0.0)	1 (5.3)
Time to activity, months	7.7±1.7	5.9±1.2	0.006*
Complications			1.000
Malunion with coxa vara	1 (5.6)	1 (5.3)
Leg length discrepancy	1 (5.6)	2 (10.5)
Avascular necrosis	0 (0.0)	0 (0.0)
Refracture	1 (5.6)	0 (0.0)

Data were presented as mean±sd or n (%).

^*Statistically significant p<0.05.

^†Fisher’s exact tests.

MSTS, Musculoskeletal Tumor Society.

According to the MSTS scores, 48.7% (18/37) of the patients had excellent, 18.9% (7/37) had good, 16.2% (6/37) had fair and 16.2% (6/37) had poor results. There was no significant difference in clinical outcomes between the two groups (p=0.896). Seventeen (46.0%) patients presented as healed and 8 (21.6%) patients presented as healed with residual. Recurrence occurred in 11 (29.7%) patients, including 6 in the internal fixation group and 5 in the group without internal fixation. Mean time to recurrence was 11.5±3.3 months (range, 7.0–18.0 months). Two patients with recurrence were reoperated. One (2.7%) patient in the internal fixation group showed no radiographic evidence of improvement and presented with no response. There was no significant difference in radiological outcomes between the two groups (p=0.657). Fair and poor results (12/37) were attributed to recurrence (11/37) and no response (1/37). Time of all 37 patients from surgical treatment until the patient was permitted to return to activity was 6.8±1.7 months (range, 5.0–12.0 months). Patients who were treated with internal fixation were able to return to activity significantly sooner than those who were not (5.9 months vs. 7.7 months; p<0.05). Complications included two cases of malunion with coxa vara, three cases of leg length discrepancy and one case of refracture; this accounted for 16.2% of all patients. There were no documented cases of avascular necrosis. There was no significant difference in complications between the two groups (p=1.000).

Discussion

The proximal femur is one of the most common locations for UBC in children, second only to the proximal humerus.¹² As the involved cortex is as thin as crushed eggshell, the potential risk of pathological fracture is increased. Aggressive surgical treatment is recommended for the proximal femur UBC.¹³ This approach involves how to address the tumor, reconstruct stability of the proximal femur and avoid possible complications. Theoretically, the greater the degree of complete cyst resection, the lower the recurrence rate.¹⁴ In practice, intralesional curettage, including drainage of cyst fluid to reduce pressure, removal of the fibrous membrane on the inner surface to destroy the intra cyst environment and subsequent filling of the bone defect cavity with bone graft to promote osteogenesis, is always proposed for UBC lesion management. Many treatment outcomes for proximal femur UBCs have been reported and recurrence rates vary greatly with different strategies.15,17 Maximen et al.¹⁸ reported a 33% recurrence or no response at all in an intramedullary nail insertion group, 57% in the curettage and a bone grafting group and 70% in a percutaneous injection group. Given the very high recurrence rate, simple percutaneous injection is not recommended as a treatment option for proximal femoral UBC.¹⁹ Surgical revision is performed in cases of recurrence. In our study, patients treated with curettage and bone grafting with or without internal fixation presented a relatively high recurrence rate (29.7%); this was lower than 36% reported by Lin et al.²⁰ The overall rate of excellent and good MSTS scores was 67.6% in our study. There was no significant difference in radiological or clinical outcomes between the internal fixation group and the non-internal fixation group. These outcomes provide important reference for the prognosis of proximal femur UBC in children. Potential risk of recurrence and unsatisfactory function should be promptly communicated to parents or guardians to establish reasonable expectations and facilitate rational decision-making.

In terms of proximal femur UBC, reconstructing stability has equal priority to addressing the tumor. Based on factors such as cyst size, the residual cortical bone of the lateral proximal femur, the extent of bone loss in the femoral neck and skeletal maturity, various reconstruction strategies, encompassing hip spica casting, traction and internal fixation, have been individually suggested.¹³ No significant difference was observed in time to cyst healing between the internal fixation group and the non-internal fixation group in our study. Interestingly, larger cystic lesions were found in patients in the internal fixation group. Determining the time to return activity is very subjective. When postoperative pain had completely subsided and radiological evidence of obvious opacification with cortical thickening or osteotylus formation in at least three cortices was observed, patients were allowed to commence partial weight bearing. Patients in the internal fixation group were permitted to return to full activity once the cyst had almost healed; this was significantly earlier than that in the non-internal fixation group. This finding suggests that selection of suitable internal fixation is beneficial in accelerating the healing process and reducing the time to activity at specific locations of the proximal femur UBC. Currently, various internal fixation alternatives, ranging from K-wires to cannulated screws and from intramedullary nails to side plates, are available.21,23 Erol et al.¹³ recommended the use of intramedullary nails to stabilize fractures for medium-UBC to large-UBC cysts in children. Moreover, a unique advantage of intramedullary nails is that they allow continuous decompression of the cyst and promote healing. However, inadequate support of the proximal femur and potential penetration of the femoral neck are disadvantages that result in a loss of fracture reduction. Adas et al.²² demonstrated that a pediatric hip plate was a reliable option for internal fixation in patients with acute pathologic fractures of the proximal femur, resulting from UBCs. This internal fixation approach directs upper screws toward the posterior-medial metaphyseal zone and lower screws to the lateral buttress of the proximal femur. This has clear advantages for restoring the neck-shaft angle and preventing loss of fracture reduction. However, excessive surgical trauma is an inevitable drawback. In our study, intramedullary nails and pediatric hip plates were used as internal fixation devices.

Proximal femur UBCs are adjacent to the growth plate and potentially interfere with the blood supply to the femoral head. This disrupts the strength of the weight-bearing lower extremity, resulting in complications, including malunion with coxa vara, leg length discrepancy, avascular necrosis and refracture. Lin et al.²⁰ reported a complication rate of 71%; this was higher than the rate of 16.2% in our study. This can be attributed to the inclusion of other variables, including loss of reduction, non-union, pathologic lesion recurrence and progression. Shrader et al.²⁴ reported a 40% complication rate in their cohort with most of those complications being leg length discrepancies. In our study, leg length discrepancies were the most common complication, but the incidence (8.1%) was not high. This might be explained by an injury to the growth plate as a consequence of cyst location or the surgery. We also found the rate of malunion with coxa vara at 5.4%; a figure of 14% was reported by Lin et al.,²⁰ which may be attributed to the higher proportion of internal fixation. Of note, we observed no avascular necrosis in our cohort. The absence of avascular necrosis in our cohort may be attributable to the small number of patients and insufficient follow-up. In our study, no difference was found between the two groups. Notably, larger cysts in the internal fixation group theoretically increased the risk of complications in these patients.

Our study has several limitations. First, information and selection biases were inevitably introduced due to a retrospective design. Second, sample size was small, and the follow-up time was too short for skeletal maturation. Third, the selected assessment systems may be inconsistent with some studies, which would have a direct effect on healing. Large multicenter studies with long follow-up periods should be conducted on proximal femur SBCs in the future.

In conclusion, although proximal femoral UBC displays benign characteristics, the major concern of surgical treatment is relatively high recurrence. Our findings suggest that for larger cysts treated by curettage, bone grafting with internal fixation requires less time to return to activity than do smaller cysts treated by curettage or bone grafting without internal fixation. Both patients and their legal guardians need to be informed of potential risks and complications.

Data availability statement

Data are available upon reasonable request.