Radioulnar synostosis is an uncommon condition where an abnormal connection of bone or fibrous tissue forms between the radius and ulna. This fusion restricts the forearm’s ability to rotate, which can interfere with a wide range of daily activities. This condition can be present from birth (congenital) or develop later in life due to injury (acquired).
Causes and Types of Radioulnar Synostosis
Radioulnar synostosis is categorized into two main types based on its origin: congenital and acquired. Each type has distinct causes and developmental pathways that lead to the fusion of the forearm bones.
Congenital Radioulnar Synostosis
Congenital radioulnar synostosis is present at birth and occurs because the radius and ulna fail to separate during fetal development. A baby’s arms develop between the fifth and eighth week of pregnancy, a period when the radius and ulna are initially connected. If this separation process is incomplete, the baby is born with the bones fused. While many cases occur for unknown reasons, genetics can play a role, with some instances following an autosomal dominant inheritance pattern. This means a child of an affected parent has a 50% chance of inheriting the condition.
This form of synostosis is sometimes associated with broader genetic syndromes, such as Klinefelter, Apert, and Holt-Oram syndromes. Mutations in the HOXA11 gene have also been linked to this condition. The fusion is most often located in the proximal third of the forearm, near the elbow.
Acquired Radioulnar Synostosis
Acquired, or post-traumatic, radioulnar synostosis develops after birth, as a complication of a significant injury to the forearm. It is most commonly seen after severe fractures of both the radius and ulna, particularly high-energy injuries that cause comminuted fractures. Surgical treatment of forearm fractures can also lead to this condition. Factors that increase the risk include using a single incision to repair both bones, screws that extend into the space between the bones, and delayed surgery. The formation of a bony bridge is a result of the body’s healing response to the trauma.
Symptoms and Functional Impact
The primary symptom of radioulnar synostosis is the mechanical inability to rotate the forearm. This involves the movements of supination (turning the palm up) and pronation (turning the palm down). In most cases, the forearm becomes fixed in a pronated position, with the palm facing downward. This limitation is often painless, especially in congenital cases or when the fusion is complete.
This fixed position creates considerable functional challenges. Tasks such as accepting change from a cashier, holding a bowl of soup, or turning a doorknob can become difficult or impossible. Other activities like eating with utensils, brushing teeth, and writing may also be affected.
The degree of functional deficit depends on the position of the fixed forearm. A forearm locked in severe pronation makes any activity requiring a palm-up position, like catching a ball, particularly challenging. Conversely, a forearm fixed in a supinated (palm-up) position would make tabletop activities like typing or writing difficult.
The Diagnostic Process
Diagnosis of radioulnar synostosis begins with a medical history and a physical examination. A doctor will assess the range of motion of the forearm, where the key finding is blocked active and passive rotation. This indicates a mechanical block rather than a soft tissue issue. Children with the condition may go undiagnosed until they are school-aged, when functional limitations become more apparent.
To confirm the diagnosis, imaging studies are ordered. X-rays of the forearm are the primary diagnostic tool, as they clearly visualize the bony bridge connecting the radius and ulna. In some cases, particularly when surgery is being considered, a computed tomography (CT) scan may be performed. A CT scan provides a more detailed, three-dimensional view of the bony anatomy, which allows surgeons to precisely map the fusion for preoperative planning.
Management and Treatment Approaches
The management of radioulnar synostosis is tailored to the individual, focusing on the severity of the functional impairment. Treatment strategies range from non-surgical observation to complex surgical procedures. The primary goal is to improve the functional position of the arm, not necessarily to restore full rotation.
Non-Surgical Management
For individuals with mild cases where the forearm is fixed in a functional position, observation is a common approach. Non-operative management is often recommended when functional deficits are minimal and individuals have developed compensatory movements. Occupational therapy plays a part in non-surgical management. Therapists can teach adaptive strategies and provide tools to help with daily tasks, improving their quality of life without invasive procedures.
Surgical Intervention
Surgery is reserved for individuals with severe functional limitations, especially when both arms are affected or the forearm is fixed in a non-functional position (greater than 60 degrees of pronation). The most common surgical procedure is a derotational osteotomy. In this operation, the surgeon cuts the fused bone and rotates the forearm into a more neutral and useful position before fixing it in place. Attempts to resect the bony bridge and restore motion have a high failure rate, as the bone often regrows. Risks associated with surgery include nerve injury, compartment syndrome, and recurrence of the fusion, and intensive rehabilitation is important to maintain the new position.