This study provides a framework for the systematic evaluation and management of the FD-induced-deformity. The spectrum of femoral deformities in full-length was classified into five categories. Furthermore, their corresponding management was recommended. Currently, there have been only two prior systematic categories[8, 11], which classify a variety of proximal femoral deformities in FD. However, both classifications were failed to guide the most appropriate strategy for treatment. In Ippolito et al’s study, three orthopedic surgeons and one pathologist evaluated FD femurs on two occasions with an interval of 6 weeks. The intraobserver (0.855) and interobserver (range, 0.833–0.871) agreement were both excellent. The highest percentage of mistakes was made when distinguishing mild shepherd’s crook deformity from severe shepherd’s crook deformity. In our primary classification system, two senior orthopedic surgeons evaluated the cases for two rounds with a 6-week interval. The intraobserver and interobserver agreements were both excellent. In this study, the mean interobserver and intraobserver kappa scores were 0.85 (range 0.77–0.89) and 0.85 (0.79–0.92), respectively. Although the agreement was excellent, the dispute was focused on the genu valgum between types III and IV. In some of the type IV lesions, genu valgum secondary to the restoration of the normal femoral alignment was remarkable after valgus osteotomy. Therefore, the surgeon must be prepared to manage the genu valgum in some type IV patients. Moreover, we recommend classifying these patients into type IV for correspondence surgical options (Fig. 1).
Type I lesions have no focal thinning of cortical bone and involvement of the calcar in the proximal femur, and we recommend conservative treatment with monitoring every 6 months (Fig. 2). Bone pain in FD should be discreetly assessed. Focal or weight-bearing pain may indicate an imminent or impending fracture. Physiotherapy and pain medication can be administered, including opioids and non-steroidal anti-inflammatory drugs. Besides, intravenous bisphosphonate is proposed for persistent, moderate to severe pain, even in children and adolescents[23, 24]. Moreover, denosumab, targeting RANKL that is expressed by osteogenic cells, maybe a potential treatment for bone pain caused by FD. During follow-up, no severe complications were detected in type I patients.
Type II lesions are characterized by focal thinning of cortical bone and/or involvement of the calcar, without other femoral deformities. Type II patients often have mechanical or weight-bearing bone pain, which is a signal of stress or an impending fracture[10, 26]. Therefore, curettage, bone graft, and internal IF are recommended (Fig. 3). The efficacy and complications of bone graft are still controversial[10, 27]. However, cortical allografts were still recommended for the final and slow internal replacement by the host bone, especially in monostotic FD patients[3, 5]. The intramedullary lesion should be adequately bridged by IF including dynamic hip screw (DHS), anatomical plate, or intramedullary nail[4, 28, 29]. In our study, only one type II patients had mild-to-moderate pain postoperatively.
Type III patients refer to the deformity of coxa vara and/or femoral shaft deformity associated with bone pain. For the single-level osteotomy site, the subtrochanteric region[5, 6, 30] (Fig. 4) and the dome of the deformity[6, 31, 32](Fig. 5) were recommended. However, the double-level osteotomy is strongly considered inadequate for correcting severe deformity[6, 31, 33]. After osteotomy, orderly curettage, massive impaction allograft, and IF are performed[4, 34, 35]. The choice of IF is still controversial. Previously, some authors suggested the longer DHS rather than intramedullary nailing, because of its ability in correcting varus and rotational deformities of the femoral neck and simplify procedure[5, 30, 32]. However, some studies report that intramedullary nails can provide good biomechanical support[6, 34]. In general, we recommend intramedullary nail for the following reasons. Firstly, it provides sufficient stability that prevents stress fracture and screw loosening or pullout, especially in polyostotic FD patients[5, 29]. Secondly, it accommodates multiple-level osteotomy. To increase the initial stability, the transversal surface of the femur after osteotomy should be entirely matched for locking each other. Additionally, the intramedullary lesions with sclerotic rim have sufficient bone mass, which provides adequate stability for nail fixation. Therefore, the sclerotic bone should be discreetly preserved when curettage and reaming canal. Moreover, rotational deformities of the femur could be gradually corrected. In our study, two type III cases still complained of pain, remained pain, owing to mild hip joint degeneration. One type III patient had mild-to-moderate Trendelenburg gait. Preoperatively, the neck-shaft angle of this patient was only 75°. Over-tensioning of the gluteus medius was inevitable postoperatively after the correction of coxa vara.
Type IV are detected in patients with severe genu valgum. When combined with type III deformity, a two-stage treatment was recommended. Complex femoral deformities and lesion curettage were corrected firstly. After six months of rehabilitation, the second-stage procedure was performed for patients, who still complained of typical symptoms. We suggest HTO or DFO, for achieving a satisfactory appearance, correcting limb alignment, and relieving pain[18, 37]. However, for polyostotic FD patients, genu valgum is mainly caused by the proximal tibia and distal femur; thus, DFO and HTO are both recommended (Figs. 6 and 7). Type IV lesions are uncommon and the most challenging type and patients with this type of deformity have a lower Guille score, compared to other types of patients. In our study, only one type IV patient had mild Trendelenburg gait, because of the over-tensioning of the gluteus medius.
Type V lesions are found in FD patients with severe hip arthritis, combined with other types (II/III/IV). The deformities of the femoral shaft, coxa vara, and genu valgum, are significantly associated with the degeneration of the hip and knee. Additionally, polyostotic FD is found to be more prevalent in hip arthritis than monostotic FD[38, 39]. In our study, type V patients had significantly higher Guille scores than type III and IV, but no significant difference was found between patients with type V and those types I and II. Although a small fraction of FD patients with mild deformity may be classified into type V, the relatively high Guille score was preliminary evidence to guide treatment. Sierra et al. firstly reported THA in patients with FD. When hip arthritis has been severe enough, total hip arthroplasty is suggested. Furthermore, a cemented stem was suggested to have a lower revision rate than an uncemented stem. However, our institution reported that the long uncemented stem showed reliable fixation at mid-term follow up, using Mimics V17.0 Software (Materialise Corp. Belgium), precise preoperative planning, and simulation of the osteotomy. Also, implantation of the prosthesis stem in a three-dimensional reconstructive model is essential. For the femoral component, we recommend fully coated stems, which engage in normal diaphyseal bone bypassing the lesion areas at least 2 femoral canal diameters to decrease the risk of postoperative fractures (Figs. 8 and 9). The stem implantation is a crucial point. First, precision osteotomy was assisted by a patient-specific instrument, using a micropendulum saw. Second, the femoral cavity was temporarily fixed after thoroughly evacuation and bone graft. Third, the femur was reamed following addressing the greater trochanter. Two type V patients had mild limping, because of leg length discrepancy after THA.
There were several limitations to our study. Firstly, the classification was retrospective and confined to radiographic images. Secondly, the follow-up time was significantly different among the five groups. Thus, multicenter studies with a longer follow-up time are needed to make sufficient conclusions on our classification, especially for type V. Thirdly, surgical strategies and classifications are appropriate only for adult patients since most patients in our department were adults (median age 29 years, range 7.1–73 years). Finally, although many patients firstly visited the surgical department due to a pathologic fracture, we have treated some patients with conditions that may not be classified by our system.