I. Imaging Findings

Based on the provided anteroposterior and lateral X-ray views of the knee, along with corresponding CT and MRI images, a well-defined low-density lesion (radiolucent on X-ray) can be seen in the proximal metaphysis of the tibia. The lesion appears oval-shaped, with localized cortical thinning and a conspicuous sclerotic rim around it. It may have a multiloculated appearance, and there is no obvious periosteal reaction. On CT cross-sections, the lesion is situated in the medial aspect of the proximal tibia, with the cortex intact but slightly thinned. MRI demonstrates a lesion with low to slightly low T1 signal and low to slightly low T2 signal, surrounded by a low-signal rim corresponding to sclerosis. Overall, the lesion is well confined, and there is no significant soft tissue abnormality or widespread infiltration around it.

There is currently no visible fracture line, but given the relatively large size of the lesion and the involvement of a significant portion of the cortex, there is a potential risk of pathological fracture if subjected to significant trauma. No obvious swelling or mass is seen in the surrounding soft tissue.

II. Potential Diagnoses

Based on the patient’s age, medical history (discovery of the lesion after trauma), imaging findings, and lesion morphology, possible diagnoses include:

1. Non-ossifying Fibroma (NOF): Commonly found in children and adolescents, but can persist into adulthood or be incidentally discovered. This benign fibrous lesion typically has a well-defined border with a sclerotic rim and is often located in the metaphysis of long bones.
2. Fibrous Dysplasia: On imaging, it can sometimes appear as a radiolucent area in the bone, but it often shows a “ground-glass” appearance or has less distinct borders. The typical location and patient’s age can sometimes differ from this presentation.
3. Giant Cell Tumor of Bone (GCT): Generally occurs around the metaphysis extending to the articular surface, often showing an expansile lesion with less well-defined borders than NOF. It is seen in younger adults and must be considered, but it usually presents with more aggressive radiographic features.
4. Aneurysmal Bone Cyst (ABC): Often exhibits an expansile, cystic lesion with septations and a “blown-out” appearance. The sclerotic margin is generally less sharp than that seen in non-ossifying fibroma.

The presence of a sclerotic margin, multiloculated radiolucency, and an absence of obvious aggressive features strongly suggest that Non-ossifying Fibroma is the most likely diagnosis.

III. Final Diagnosis

Considering the patient’s age (26 years old), incidental finding of the lesion after trauma, the typical shape and radiographic appearance, and the lack of notable pain or swelling, the final diagnosis is most likely “Non-ossifying Fibroma (NOF)”. This is a benign lesion that usually does not require biopsy. If successive follow-up imaging does not reveal any malignant indicators, this diagnosis can be confidently confirmed.

IV. Treatment Plan and Rehabilitation Scheme

1. Treatment Strategy

For small, asymptomatic non-ossifying fibromas, no specific treatment is required, and periodic follow-up is generally sufficient. However, if the lesion diameter exceeds 3–3.3 cm or involves most of the bone shaft (more than 50% of its transverse diameter), there is an increased risk of pathological fracture. In such cases, one may consider:

• Curettage of the Lesion: Thoroughly curette the lesion and fill the defect with autograft or allograft bone to enhance bone strength and prevent or address fractures.
• If a Pathological Fracture Occurs: Initial conservative fixation with a cast or brace is recommended. Further surgical intervention can be decided based on fracture healing and the size of the lesion.

2. Rehabilitation and Exercise Prescription

Rehabilitation should follow a gradual, individualized approach:

• Initial Phase (early postoperative or observation period): Emphasize protective or partial weight bearing, avoid high-impact activities (e.g., running or jumping) if not operated on, and perform light functional exercises (such as straight leg raises and isometric quadriceps contractions) to maintain muscle tension and joint flexibility.
• Intermediate Phase: Depending on assessments by physicians and physical therapists, and provided bone healing and lesion stability are confirmed, gradually increase lower limb strengthening exercises (e.g., resistance band exercises, seated or partial squats). Pain and local discomfort should be closely monitored.
• Advanced Phase: Once sufficient bone strength is restored, progressively incorporate higher-load or dynamic balance exercises (e.g., squat training in a closed kinetic chain, single-leg balance exercises) and transition to low-impact aerobic activities (e.g., stationary cycling, elliptical training). Eventually, activity can be escalated to meet normal daily and sports demands.

For exercise planning, refer to the FITT-VP principle (Frequency, Intensity, Time, Type, Volume, Progression):

• Frequency (F): 3–5 times per week, integrated with daily activities.
• Intensity (I): Begin with relatively low intensity (30%–50% of maximum muscle strength), progressing to 50%–70% in later stages.
• Time (T): 20–30 minutes per session, adjustable based on tolerance.
• Type (T): Early focus on range-of-motion maintenance and basic lower limb strengthening. Later, include aerobic exercises and balance training.
• Progression & Volume (P / VP): Increase the difficulty and total exercise volume gradually, contingent on bone healing and the absence of symptoms.

Throughout the rehabilitation process, avoid excessive loading or repetitive impact to prevent fracture risk from stress concentration. If severe pain, swelling, or other adverse symptoms occur, seek prompt re-evaluation.