After an injury of a joint an 8 year old child developed stiffness of the joint. Development of myositis ossificans is seen in X-ray. The joint commonly involved is:

Myositis ossificans or post-traumatic ossification is the process of ossification within muscle occurs when sufficient trauma causes a hematoma that later heals in the manner of a fracture. Contusions of the quadriceps of the thigh or the triceps of the arm (around the elbow) are the most common injuries. Disability is great, with local swelling, heat and extreme pain with the slightest provocation of the adjacent joint. The limb should be rested until the local reaction has subsided (5-7 days). Ref: Erickson M.A., Merritt C., Polousky J.D. (2012). Chapter 26. Ohopedics. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.