Allogenic Cancellous Bone versus Injectable Bone Substitute for Endoscopic Treatment of Simple Bone Cyst and Intraosseous Lipoma of the Calcaneus and Is Intraosseous Lipoma a Developmental Stage of a Simple Bone Cyst?

Abstract

Simple bone cysts (SBCs) and intraosseous lipoma (IOL) of the calcaneus are rare tumor entities that are primarily diagnosed due to unspecific heel pain, incidental findings, or rarely due to pathological fractures. Compared to traditional open tumor resections, endoscopic resection of these benign tumors aims to minimize surgical morbidity and maximize surgical efficiency without compromising safety. Grafting is regularly performed to reduce the risk of recurrence and stimulate osseous consolidation of the lytic lesion. As the incidence is low and treatment strategies are heterogeneous, there is no clear consensus for the treatment of simple cysts or intraosseous lipomas of the calcaneus. The objectives of this study are (a) to present medium to long-term results after endoscopic resection and grafting with allogenic cancellous bone or bioresorbable hydroxyapatite and calcium sulfate cement, and (b) to add further evidence to the discussion of whether calcaneal SBC and IOL are the same entity at different developmental stages. Between 2012 and 2019, a total of 25 benign bone tumors consisting of 17 SBCs and 8 IOLs were treated by A.T. with endoscopic resection and grafting, comprising the largest cohort to date. For grafting, 12 patients received allogenic cancellous bone (group A) and 13 patients received injectable bone substitute (group B). Pre- and postoperative imaging using plain X-rays and MRI was retrospectively analyzed with a mean follow-up time of 24.5 months to assess tumor size, osseous consolidation (modified Neer classification), and tumor recurrence. A retrospective chart analysis focusing on adverse intra- and perioperative events and other complications associated with the surgical procedure was performed using the modified Clavien-Dindo classification (CD1-3). A total of 12/13 cases with allogenic bone grafting showed a Neer Type 1 osseous healing of the tumorous lesion after endoscopic resection, whereas only 5/11 cases with injectable bone substitute showed sufficient healing (types 1 and 2). There were three recurrent cysts (Neer 4) and two persistent cysts (Neer 3) after using injectable bone substitute. Two CD1 complications were observed in group A (prolonged wound drainage, sural neuritis) and eight complications were observed in group B (6× CD1, 2× CD3). At least two IOLs diagnosed preoperatively using MRI were ultimately identified as SBCs upon histopathologic examination. Allogenic cancellous bone grafting after endoscopic resection of calcaneal SBC or IOL showed a very low rate of complications and no tumor recurrence in our series. On the other hand, depending on the material used, injectable bone substitute showed a high rate of "white-out" (excessive drainage), resulting in multiple complications such as prolonged wound healing, insufficient permanent defect filling, recurrence, and revision surgery. Over time, calcaneal SBC may transform into IOL, exhibiting distinct features of both entities simultaneously during ossoscopy and histopathological analysis.

Keywords: allografting; benign bone tumor; calcaneal bone cyst; endoscopy; injectable bone substitute; intra-osseous lipoma; lipoma of bone; ossoscopy; simple bone cyst; white-out.

Figure 1

Schematic representation of the calcaneal trabecular structures according to the classification of Diard et al. [3]. The trabecular bone architecture of the calcaneus is formed by five trabecular groups, with the first three main trabecular groups defining a central area characterized by reduced bone density and increased radiolucency (Diard’s Area 6) [3,23].

Figure 2

After the introduction of the scope into the bone cavity, vision was often impaired by fat tissue in the case of IOL (a). After thorough irrigation and endoscopic removal of the fat tissue using an arthroscopic shaver, residual calcifications were removed using an arthroscopic grasper or shaver (b–d).

Figure 3

Often, typical features of IOL ((a), fat tissue) and SBC ((b) membrane/cyst lining) were both observed in parallel in calcaneal ossocopy, suggesting a common pathogenesis and transitional forms of SBC and IOL. Both images are from the same ossoscopy (case 12).

Figure 4

A Hartmann ear septum borrowed from ENT surgery facilitated the insertion of the allogeneic cancellous bone into the bone cavity.

Figure 5

Injection of the resorbable bone substitute (in this case Cerament©) was visualized directly under endoscopic (and fluoroscopic) control.

Figure 6

(a) Axial and coronal T2 TSE-weighted MRI showing a large SBC in a 31-year-old professional ballet dancer (case 6). After endoscopic resection, the bone cavity was filled with injectable bone substitute. (b) Sagittal T1-weighted MRI performed 91 months postop, showing two small areas of recurrence of SBC at the medial aspect (bottom image, arrows). The large cyst cavity was filled with QuickSet© injectable bone substitute (marked with a star), showing no signs of osseous remodeling more than 7.5 years after implantation. The patient is currently free of symptoms and still performs at the highest level of professional ballet dancing.

Figure 7

Case 24: Persistent wound drainage resembling pus led to revision surgery 14 days after index surgery for suspected deep infection. Intraoperative microbiological testing did not prove infection. “White-out” after injection of Cerament© was identified as the cause of the complication.

Figure 8

Preoperative MRI of case 12 (19-year-old male patient) showing a calcaneal IOL with cystic changes corresponding to a Milgram Stage 3 type [29]. However, histopathological examination of the tissue samples taken from the bone cavity revealed SBC.

Figure 9

Top, left to right: (a) preoperative X-ray of a large IOL, Milgram Stage 3 (case 22); (b) intraoperative fluoroscopy, postoperative radiographs at 6 (c) and 12 (d) weeks postoperatively showing continued loss of the filling material, consistent with persistent wound drainage. Bottom, left to right: (e) preoperative MRI, (f) sagittal MRI 12 months postop, (g) 20 months postop, and (h) 32 months postop. All MRI are sagittal T1 TIRM weighted.

Figure 10

Conventional radiological and MR tomographic follow-up of case 23: (a) preop, (b) 3 days postop, (c) 8 weeks postop, and (d) 3.5 months postop. Leakage of Cerament© was observed clinically (“white-out”) and radiologically.

Figure 11

MR-imaging of case 26: (a) preop (PD TSE sag), (b) 6.5 months postop (T1 TSE sag), (c) 13 months postop (T1 TSE sag), and (d) 40 months postop (T1 TSE sag), showing no signs of osseous remodeling after more than 3 years postoperatively.