To Cut, or Not to Cut: Evaluation of Bone Formation Activity and Bone Viability Using 3-Phase Bone Scintigraphy

Introduction: 99mTc-methylene diphosphonates (MDP) 3-phase bone scintigraphy is an excellent imaging modality in the evaluation of bone formation activity and bone viability. Even though the image acquisition techniques and use of the radiopharmaceuticals are mostly the same, interpretations of the images in the clinical context are very different. Misinterpretation or misleading statements can have a detrimental effect on the patient.

Methods: The purpose of this educational exhibit is to present cases that are not so common but crucial in clinical decision making.

Results: 1. Mandibular condylar hyperplasia (MCH): Cut the hot part out. Leave the cold part as is. (Fig. 1)

MCH is the most common cause of asymmetry of the jaw. Osteoblastic activity determines which surgical method to be performed. When the mandibular condyle is in the active phase, high condylectomy is the surgical option. Resection of an inactive condyle causes unnecessary disruption of the TMJ and can later manifest as malocclusion. Orthognathic surgery is performed with inactive condyle which involves correcting mostly of the mandibular body and ramus but not the condyles. Leaving the active condyle after correcting the jaw will only cause a recurrence of the jaw asymmetry.

2. Heterotopic bone formation (HBF): Wait for the hot to cool down. Cut when it gets warm. (Fig. 2)

HBF is a pathologic condition in which bone tissues exist in soft tissue where they normally should not be. Hypervascularity is one of the important mechanisms in pathophysiology, especially in the subacute stage. Resection of the active and immature heterotopic bone tissue will highly likely to grow back. It is recommended to do surgical removal when the HBF is matured. The bone scan will show a return of the uptake to normal or show a sharp decreasing trend followed by a steady state for 2-3 months.

3. Bone viability: Save the remaining embers. Cut the cold part out. (Fig. 3)

Bone viability is assessed in various clinical scenarios especially when there is an impending chance of amputation. Frostbite, burns, diabetic osteomyelitis, bone graft, and avascular necrosis are examples among the many scenarios that can be indicated. A good blood supply is essential for tissue viability, which can be assessed by angiography or MRI. However, patency of the vessels alone does not suffice, and seemed to be insufficient blood flow to a certain area might still maintain bone viability most likely from collaterals.

Conclusions: Bone scintigraphy provides crucial functional assessment of bone formation activity and viability in various clinical settings. Appropriate guidance based on appropriate clinical context should be delivered to the surgeons.

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