1 Introduction

Isolated femoral condylar fractures are rare and accounting for approximately 0.65 % of all fractures in femur.¹ Subchondral insufficiency fractures of the knee occur under physiological loads with decreased mechanical bone resistance, typically in the weight-bearing area of the medial femoral condyle. On the other hand, subchondral insufficiency fractures at the non-weight-bearing surfaces of the femoral condyles are quite rare. Only one study has reported such fractures, called ‘Prayer's fracture,’ which occurred in five patients who performed daily Muslim prayer rituals that involved repetitive high knee flexion.² Repetitive deep knee flexion may cause transient changes in the weight-bearing zone, resulting in an unusual fracture location. In these cases, the fracture site included the medial condyle in one and the lateral condyle in four cases.

Herein, we aimed to present a case of a subchondral insufficiency fracture in a non-weight-bearing area of the lateral posterior femoral condyle (LPFC).

2 Case report

An 82-year-old woman (height, 138 cm; weight, 35 kg; body mass index, 18.4 kg/m²) was initially diagnosed with left femorotibial knee osteoarthritis and treated conservatively for 8 months at her previous hospital. The patient was transported to our hospital by ambulance because of severe knee pain that prevented her from standing and walking. The patient denied a recent history of knee trauma. The patient had no medical history of knee surgery, medication use, diabetes mellitus, arthritis, or systemic diseases, except for a lumbar compression fracture that was surgically treated with lumbar fusion 10 months before presentation.

Range of motion of her left knee was 5–120° preoperatively. Pre-operative anteroposterior full-length lower limb radiological evaluation revealed a slightly narrow joint space on the tibiofemoral joint, with a femorotibial angle (FTA), the lateral angle between the femoral and tibial shaft axes, of 170° (Fig. 1A). The weight-bearing line (WBL) refers to the line drawn from the centre of the femoral head to that of the talus dome. The WBL ratio of the tibial width was 57.9 %. Lateral and axial radiographs of the distal femur³ revealed a bone defect in the LPFC (Fig. 1B and C). Similarly, computed tomography (CT) images elucidated a collapse of the centre of the LPFC (Fig. 2A–C). Magnetic resonance imaging (MRI) revealed flattening of the subchondral bone of the non-weight-bearing aspect of the LPFC without adjacent marrow edema (Fig. 2D–F).

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Fig. 1 Pre-operative anteroposterior, lateral, and axial radiographs of the distal femur. (A) Pre-operative anteroposterior full-length lower limb radiograph revealed a slightly narrow joint space on the tibiofemoral joint, with a femorotibial angle of 170° (*). The weight-bearing line ratio was 57.9 % (↑). (B, C) Lateral and axial radiographs of the distal femur revealed a bone defect of the posterior lateral condyle (↑).

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Fig. 2 Pre-operative computed tomography (CT) images and Magnetic resonance imaging (MRI) of the knee. (A, B, C) CT images with (A) sagittal, (B) axial planes, and (C) three-dimensional image depicting the collapse of the posterior non-weight-bearing aspect of the lateral femoral condyle (↑). (D, E) Sagittal (D) T1W and (E) T2W and (F) axial T1W MRI show flattening of the subchondral bone of the non-weight-bearing, posterior aspect of the lateral femoral condyle without adjacent marrow edema (↑).

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A left total knee arthroplasty (TKA) was undergone using a total stabilised prosthesis and patella resurfacing (Triathlon TS knee system, Stryker Orthopaedics, Mahwah, NJ). A lateral curved surgical incision was made using a parapatellar approach. A slight joint effusion was observed; however, no signs of infection were observed. The cartilage was detached from the non-weight-bearing area of the LPFC, and the subchondral bone was defective (Fig. 3A and B). The LPFC was cut parallel to the surgical epicondylar axis. Based on the pre-operative radiographic or CT evaluation, the lateral posterior condyle was also defective, as suspected. An augmentation block of 5 mm with an intramedullary stem was used for bone defects in the posterior lateral femoral condyle (Fig. 3C). All components were fixed with cement.

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Fig. 3 Intraoperative photographs of total knee arthroplasty. (A) The cartilage was detached from the posterior, non-weight-bearing area of the lateral femoral condyle (↑). (B) Defective subchondral bone appeared when the cartilage was retrieved (↑). (C) A 5 mm augmentation block with an intramedullary stem was used for bone defects in the posterior aspect of the lateral femoral condyle.

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Post-operative radiographs revealed an FTA of 177° with no component malalignment (Fig. 4). The patient experienced an uneventful hospital stay and rehabilitation period. Twelve months post-operatively, the patient had a range of motion between 0° and 115° without any pain. Her Knee Society score improved from a pre-operative score of 34–92 post-operatively. The knee injury and osteoarthritis outcome score improved from a pre-operative score of 136–316 post-operatively. Bone mineral density (BMD) was 0.220 g/cm² in the non-dominant forearm (DTX-200, Toyo Medical, Tokyo, Japan), leading to a diagnosis of osteoporosis. Denosumab (Daiichi-Sankyo, Japan) was administered to treat osteoporosis.

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Fig. 4 Post-operative radiographs 1 year following the total knee arthroplasty. (A) Anteroposterior, (B) lateral radiographs.

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The histological findings of the retrieved cartilage and bone of the LPFC indicated subchondral insufficiency fractures without any specific findings or osteonecrosis.

3 Discussion

Grzelak et al.⁴ observed impaction fracture in the posterior-superior region of the non-weight-bearing area of the medial femoral condyle in five representatives of the Polish national Olympic weightlifting team. The authors hypothesised that accumulating microtraumas during weightlifting-specific activities caused this type of subchondral impaction fractures. They also proposed two mechanisms of trauma that might have occurred at the medial condyle in their activities: (i) impaction of the medial condyle in the full squat position and (ii) repetitive pressing of the medial condyle with the joint fully weighted against the slope of the extension facet during the split jerk. In contrast, Depasquale et al. reported 16 cases of non-weight-bearing impaction fractures of the LPFC on MRI.⁵ They demonstrated that the non-weight-bearing surface of the LPFC hinges on the popliteal, tibial surface during flexion, that is, direct impaction contacts of the femur and tibia in the lateral joint compartment. However, the congruency of the medial articular compartment is maintained. The knee flexion leads to gradual posterior translation of the contact point of the lateral femoral condyle. Nakagawa et al. have also demonstrated that the non-weight-bearing area of the lateral femoral condyle was 7 ± 5 mm posterior to the posterior tibial cortex at full flexion, and the lateral femoral condyle olny touch the lateral tibial condyle.⁶ An isolated coronal plane fracture of the posterior femoral condyle is known as Hoffa fracture.⁷ Rare fractures are usually related to high-energy trauma injuries such as motor vehicle crashes, which exert an axial load on a flexed knee and commonly involve the lateral condyle. The valgus physiology of the normal knee joint might cause an abduction force against the tibial plateau, resulting in a higher frequency of lateral condylar fractures.^1,8 The nature of a Hoffa fracture differs from that of the fracture reported in our case; however, the injury location is similar. Our patient had valgus alignment pre-operatively, 170° FTA and a 57.9 % WBL ratio, which might have caused a subchondral insufficiency fracture of the LPFC.

Depasquale et al. presented that out of 16 patients with a mean age of 40, ranging from 21 to 60 years, eight cases were related to recreational sports activities, four with occupations involving repeated squatting or kneeling, and one patient who was on internal contraceptives had low BMD.⁵ Considering the similar incidence rates between the sexes, the authors suggested that the risk of this fracture is more associated with patients' activity rather than sex. Our patient had no habit of repetitive kneeling; however, a full flexion, including the ‘‘Seiza’’ position, sitting in a kneeling position, is often used in daily life in Japan. Furthermore, the patient was a woman with low BMD and physical activity because of the recovery period after surgery for lumbar compression fractures. In addition, her Japanese lifestyle might have contributed to this fracture.

In previous studies, diagnoses were based on imaging^2,4,5; however, we confirmed the fracture histologically in addition to the imaging findings. This unique fracture type can cause knee pain and may be difficult to diagnose. It could be missed on anteroposterior radiographic evaluations. Observation of lateral and axial radiographs of the distal femur or CT or MR images can help understand the distal femoral anatomy and evaluate the fracture type.

4 Conclusion

Knee subchondral insufficiency fractures of the non-weight-bearing portion of the LPFC are rare. This is the first report on the surgical treatment and histological evaluation of a subchondral insufficiency fracture at this location. We demonstrated the successful management of this case with TKA, using an augmentation block and intramedullary stem. We discovered that sex, low BMD and physical activity, and lifestyle might have caused the fracture.

Financial Support and sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Informed consent

The patient was informed that data concerning her case would be submitted for publication, and gave her consent.

Ethical committee approval

Not applicable.

CRediT authorship contribution statement

Hideo Kobayashi: Writing – original draft. Yasushi Akamatsu: Writing – original draft. Ken Kumagai: interpretation. Yutaka Inaba: interpretation, All authors read and approved the final manuscript.