Hemicortical Resection and Biological Reconstruction using Allograft for Parosteal Osteosarcoma of the Distal Femur. A Case Series

Olga D Savvidou (  olgasavvidou@gmail.com ) National and Kapodistrian University of Athens, ATTIKON University General Hospital Stavros Goumenos National and Kapodistrian University of Athens, ATTIKON University General Hospital Ioannis Trikoupis National and Kapodistrian University of Athens, ATTIKON University General Hospital Angelos Kaspiris University of Patras Dimitra Melissaridou National and Kapodistrian University of Athens, ATTIKON University General Hospital Panagiotis Gavriil National and Kapodistrian University of Athens, ATTIKON University General Hospital Jimmy Georgoulis National and Kapodistrian University of Athens, ATTIKON University General Hospital Panayiotis J Papagelopoulos National and Kapodistrian University of Athens, ATTIKON University General Hospital


Background
Osteosarcomas are high grade intramedullary bone tumors that require wide excision and endoprosthetic reconstruction for limb salvage surgery. Parosteal osteosarcomas (PAO), that belong to surface osteosarcomas, compose rare distinct clinicopathological category of osteogenic tumors rather than a subtype of intramedullary conventional osteosarcomas [1].
PAO are the most common surface tumor, representing 6% of all osteosarcomas and have a predominance for the second, third and fourth decade of life as well as in female gender. The posterior cortex of the distal femur is the most commonly affected site while it can also be detected in the metaphysis of other long bones [2].
Wide tumor resection and endoprosthesis or biological reconstruction are the most commonly used therapeutic strategies [2]. PAO are low-grade tumors with low risk for metastasis and local recurrence, hence PAO of the distal femur can be managed with hemicortical wide tumor excision and biological reconstruction, reducing the risk of endoprosthetic reconstruction complications [3].
The purpose of this study was to evaluate the oncological and functional outcomes of hemicortical excision and biological reconstruction using allograft for the treatment of PAO of the posterior aspect of distal femur.

Methods
The study was approved by the ethical committee of ATTIKON University General Hospital with the reference number of AD 232/19-04-2021. All patients agreed to participate in the study and provided written consent prior to publication. The research complies with the 1964 Helsinki Declaration and its later amendments.
From 2010 to 2018, 11 patients with PAO of the posterior cortex of the distal femur underwent wide hemicortical resection and reconstruction with hemicortical strut allograft in our Department. There were 7 female and 4 males. The mean age and BMI of the patients was 29 ± 7.46 years and 25 ± 2.76 kg/m 2 , respectively. All patients were diagnosed with well differentiated PAO of the distal femur, involving less than 60% of the cortex circumference, and based on computer tomography (CT) studies, magnetic resonance imaging (MRI) and histopathological results, intramedullary extension was not detected. None of the patients had distant metastatic disease.
The operation performed under general anesthesia and a pneumatic tourniquet was used on the proximal thigh. After patients were set on prone position, a midline longitudinal incision at the posterior aspect of the distal thigh and knee was performed. The fascia and the popliteal space were carefully dissected, and the peroneal and tibial nerve, as well as the femoral artery and vein were identi ed and preserved.
The posterior aspect of the distal femur was exposed and removed en-block with the tumor, using osteotomes. The mean cortical resection was 14.64 ± 5.5 cm. The posterior cortex of a distal femur allograft was prepared to match the cortex defect of the host. In 3 patients two cancellous compression screws in a posteroanterior direction were used for prophylactic xation ( Figures 1A, B-6A, B) , while in 9 patients anatomic distal femoral plate was used to secure the allograft on the host bone ( Figures 7A, B-10A, B). A drain suction was used and the wound was closed in layers. Postoperatively a posterior splint and later a brace was applied until radiographical union was demonstrated. Partial weight-bearing was started when more than 50% union of the transverse and longitudinal osteotomies appeared in the radiographic evaluation.
None of the patients were lost during the follow-up. The average postoperative follow-up was 53.64 months± 16.94 months (range, 30 to 84 months). None of the patients received neoadjuvant chemotherapy.
All surgical procedures, postoperative evaluation and assessment were performed by the same surgical team. The primary endpoint of our study was the achievement of satisfactory disease control. Secondary endpoints were: functional outcomes during the last follow-up, time to graft incorporation and assessment of postoperative complications.
As for the primary endpoint assessment, all patients underwent sequential imaging staging (MRI) and computer tomography (CT) scan for local and systematic disease progression and a CT scan of the chest every four months in the rst two years, and every six months thereafter. Regarding the secondary endpoints of our study, functional results were evaluated using the Musculoskeletal Tumor Society Score (MSTS score) [4] and measuring the knee range of motion.
The time of the allograft incorporation was established based on radiographic ndings of union. The patients were examined every month until the osteotomies had consolidated, and then after every sixmonth. Bridging across three of four cortices in biplanar radiographs was considered evidence of consolidation.
The postoperative complications were classi ed as mechanical (type 1,2,3), and non-mechanical (type 4 and 5) according to the classi cation system regarding the failure of limb salvage after biological reconstruction described by Henderson et al. [5].

Results
Wide excision of the lesion with clear surgical margins was achieved in all patients based on histopathological examination. At the follow-up examination no distant metastases were detected. All patients were free of disease at the latest follow-up evaluation (primary endpoint).
The mean MSTS score was 93.45 ± 3.56. All patients were ambulatory without any postoperative pain or restriction of daily activities at the latest follow-up. The mean range of knee exion was 105.1o ± 7.66.
Radiologic ndings showed successful union within the rst postoperative year. The average time of allograft incorporation was 7.64 ± 1.8 months.
As far as the postoperative complications, there were no soft tissue (type 1), hardware failure (type 3) or implant related infections (type 4) based on Henderson criteria. Only one patient had local recurrence (case no 6) and underwent revision surgery with wide excision and reconstruction using bula autografts ( Figures 11A, B-16A, B). This patient remains free of the disease ve years postoperative. (Table 1).

Discussion
PAO is the most frequent surface osteosarcoma. It is a low-grade tumor with low risk for local recurrence and metastatic potential. The most typical location is the posterior surface of distal femoral metaphysis, followed by proximal tibia, bula and humerus whereas involvement of the ulna, tarsal and carpal bones has been sporadically described. Symptoms include a painless mass with duration of at least one year.
As the mass increases, pain worsens. Decreased range of motion might occur if the lesion is closed to a joint [6,7].
Radiographically, PAO appear as a large lobulated mass with extensive, generally central bone formation and a thick mineralized stalk which looks to be "stuck" onto the bone surface, while the medullary cavities usually are spared. CT con rms the presence of the broad, of cortical density stuck and the cleft sign that separates the ground glass mass from the bone cortex. MRI can be used to visualize the overlying cartilaginous cap that frequently coexists.
The soft tissue component is isointense to hypointense on T1 weighted images, inhomogenous on uidsensitive MR images and shows inhomogenous enhancement on contrast-enhanced MR images; the cartilage cap is hyperintense on uidsensitive MR images whereas the osteoid matrix is hypointense on both T1w and uid-sensitive MR images [8].
Histologically, the tumor is composed of hypocellular areas of spindle cells arranged in fascicles in desmoplastic collagenous stroma with parallel trabeculae of well formed woven bone ('streamer pattern'). Spindle cells are characterized by minimal or, less frequently, moderate atypia and low mitotic activity. Its low-grade appearance and positivity for MDM-2 and CDK4 are key features in differentiating it from other surface sarcomas [8].
Wide resection of the tumor demonstrate 90% survival at 10 years follow-up [5]. Local recurrence may occur when wide resection is inadequate or in case of dedifferentiated PAO [10]. After wide tumor excision of the distal femur, the most common location of PAO, endoprosthesis or different reconstruction techniques can be used. Due to the low grade of the tumor and to the young age of the patients hemicortical resection and reconstruction of the bone defect using polymethyl methacrylate (PMMA) or biological materials such as bular autografts, allografts or pasteurized/autoclaved/irradiated host bones, with or without prophylactic xation have been reported with good oncological and functional outcome [11][12][13][14].
Endoprosthetic replacements of the femoral defect have demonstrated long term survival and good functional outcome, following resection of PAO [15]. However postoperative complication rate is high and include infection, aseptic loosening, mechanical failure, fracture of the prosthesis or of adjacent bone [2,[16][17]. Allograft allows mechanical and biological reconstruction after wide excision of tumour however, it is associated with high rates of complications, including fracture, non-union, transmission of disease and infection [18].
Campanacci et al. in 1984 was rst described [19] the hemicortical resection with unicortical window of the distal femur in the early stage of PAO of the distal femur. Later this hemicortical resection method was established as safe therapeutic procedure [11]. However, due to the rarity of the disease, there are no studies with a large number of patients and data on functional outcomes. Lewis et al. reported very good oncological and functional outcomes with no complications using this technique in 6 patients with PAO of distal femur [12]. Deijkers et al., in one of the largest cohort studies, analysed 22 allograft hemicortical reconstruction procedures, showing very satisfying oncological and functional outcome. All patients had good or excellent MSTS score, low complication rates and the allograft incorporation rate thirty months postoperative was 100% [13]. Agarwal et al. reviewed 10 patients who underwent hemicortical excision of the distal femur and reconstruction with autograft or allograft. In both methods, functional outcome was optimal and there were no major surgical complications or local recurrence [3]. Liu et al. reported very good oncological and functional outcome with relative low complication rates in 13 patients diagnosed with PAO and managed with wide excision and reconstruction with pasteurized hemicortical autograft and internal xation. The authors stated that the technique although demanding is safe and effective in selected patients [20].
Compared to endoprosthetic reconstruction after wide resection of PAO of distal femur, many studies demonstrate, respectively good functional and oncological outcome. Funovic et al. compared 12 patients who underwent prosthetic reconstruction with 11 patients who underwent biological reconstruction. The authors concluded that oncological and functional outcome does not differentiate between the two reconstruction methods, but in the endoprosthetic reconstruction group signi cantly more revisions were needed (58%) compared to the biologic reconstruction cohort (18%) at an average of 10 years [21]. Wilke et al. compared 5 patients who underwent endoprosthetic reconstruction with 7 patients who underwent biological reconstruction. After long-term follow-up, patients' group reported the same mean MSTS score (mean MSTS 23), range 16-30 in the endoprosthetic group and 18-26 in the allograft reconstruction group, while the rate of complications and reoperation was not differed between the two groups [2].
Although hemicortical allograft is a reliable biological reconstruction method after wide excision of PAO of the posterior cortex of distal femur, complications may occur. The most common complication is host bone fracture and local recurrence, followed by non-union and infection. Rarely allograft fractures can occur. These complications are associated directly to the size of bone defect after wide excision and most commonly require surgical reintervention [22]. None of our patients had a host bone fracture, allograft fracture, nonunion or infection. However, only one patient had local recurrence and was managed with revision and second time biological reconstruction with bular autograft.
The results of our study are in accordance with that of the published literature that delves into the biological reconstruction after hemicortical resection of PAO of the posterior aspect of distal femur. We acknowledge that despite the satisfying results, our study has several limitations. It is an observational, retrospective study, which included a small number of patients, without a comparison group. However, all 11 patients were treated by the same oncology team, and were operated by the same surgeon (PJP). Randomized multicenter studies with large number of patients with PAO of distal femur are needed in  Table   Table 1 is not available with this version.