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, fibula 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 confirms 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 fluid-sensitive MR images and shows inhomogenous enhancement on contrast-enhanced MR images; the cartilage cap is hyperintense on fluidsensitive MR images whereas the osteoid matrix is hypointense on both T1w and fluid-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].
Differential diagnosis included fibrous dysplasia, myositis ossificans traumatic, osteochondral exostosis, developmental defect at insertion of adductor magnus or bizarre parosteal osteochondromatous proliferation, (BPOP), juxtacortical chondrosarcoma, high grade surface osteosarcoma and parosteal lipoma [8,9].
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 fibular autografts, allografts or pasteurized/autoclaved/irradiated host bones, with or without prophylactic fixation have been reported with good oncological and functional outcome [11-14].
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 first 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 fixation. 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 significantly 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 fibular 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 order to establish these good functional and oncological results and compared them to the other reconstruction techniques.