Introduction

The term synostosis indicates osseous union between two bones that are normally separate. It may be complete, when fusion occurs throughout the opposing surfaces, or partial. It may also be categorized as longitudinal when fusion occurs between two bones lying end to end, e.g. humerus and ulna, or transverse when it involves two bones lying side by side such as the radius and ulna. In the upper extremity synostosis can be seen at the elbow, carpus, metacarpals and phalanges. Congenital radio-ulnar synostosis occurs either as an independent entity or as part of a more general condition such as complex syndactyly or ulnar ray deficiency. It can also be a part of a variety of syndromes like fetal alcohol syndrome or trisomy 13 or 21.

Background and aetiology

The upper limb bud appears as a mass of mesoderm-derived mesenchyme covered by ectoderm on the ventrolateral wall of the 4-week-old embryo. Embryogenesis occurs over the next 4 weeks and most of the upper limb structures are present by 8 weeks after fertilization. Limb development is regulated by Hox genes (Hox A, B, C and D) . Hox genes encode various transcription factors that are responsible for the aggregation of the mesenchymal cells into condensations which form the pre-cartilaginous skeletal foundation of the limb. Limb orientation along the three anatomical axes is governed by three crucial signalling centres. The three signalling centres include the apical ectodermal ridge that is responsible for the proximal to distal differentiation of the limb, the zone of polarizing activity responsible for anteroposterior limb development and the Wingless-type (Wnt) signalling centre responsible for the dorsal to ventral axis configuration.

A single mesodermal condensation is seen along the long axis of the limb bud at week 4. Pre-chondrogenic cells in these condensations then differentiate into chondrocytes under the influence of bone morphogenetic proteins (BMPs). Sequential chondrification and ossification subsequently follow, resulting in the formation of bones and joints in the upper limb. The elbow becomes visible at 34 days. The humerus, radius and ulna are continuous with each other and are joined by a common perichondrium at 5 weeks of gestation. The forearm is in a neutral position and subsequently by the 6th week condensation of the tissue separates the cartilaginous anlage of the three bones. The forearm pronates at the 8th week due to growth discrepancy between the arterial tree and the radius.²

Any insult during this period of rapid limb development can result in congenital anomalies in the upper extremity, including congenital radio-ulnar synostosis, which results from failure of longitudinal segmentation and persistence of the cartilaginous anlage. Endochondral ossification then proceeds and the cartilaginous synostosis ossifies either partially or completely in the longitudinal or transverse plane. Elbow synostosis with limb hypoplasia occurs in thalidomide embryopathy.

Although most cases are sporadic, a positive family history in some cases of congenital radio-ulnar synostosis has suggested a genetic component. Inheritance may be autosomal dominant or recessive. Some of these cases, especially humeroradial synostosis may be syndromic as they are associated with other congenital syndromes like Carpenter syndrome, Apert syndrome, Antley–Bixler syndrome, Roberts syndrome,^3,4 arthrogryposis and Cornelia de Lange syndrome. Chromosomal abnormalities such as Klinefelter syndrome⁵ may be associated with radio-ulnar synostosis.

Classification

Classically, synostosis has been divided anatomically into humeroradial, humero-ulnar, humeroradio-ulnar and radio-ulnar. Further subdivisions in the humeroradial group have been added based on the anatomical differences, namely classes 1 and 2:⁶

Table 15.1 Synostosis classification

Proximal radio-ulnar synostosis has also been classified on the basis of the radiological appearance. Mital⁷ presented a two-group classification system:

This was modified by Cleary and Omer⁸ to a four-group system:

The drawbacks of the above classification systems are that they are based purely on anatomy or radiological appearance and are not useful for evaluating the prognosis or treatment outcome. They do not indicate aetiological factors, inheritance pattern or a syndromic association.

An aetiological classification of elbow synostosis was proposed by McIntyre and Benson.⁹ They divided synostosis around the elbow into two classes:

All the three anatomical types can occur within each class, may be sporadic or familial and, if familial, may be inherited as an autosomal dominant or recessive trait. The class I group is associated with musculoskeletal abnormalities outside the affected limb in 25–55% of cases.¹⁰ Class II familial cases may be syndromic and associated with multiple synostosis syndrome, Pfeiffer syndrome, Apert syndrome and Antley–Bixler syndrome.

Presentation, investigations and treatment options

Surgical techniques and rehabilitation

Mobilization procedure¹¹

Kanaya described this procedure in the following four steps:

1. Separation of the synostosis

2. Shortening angulation osteotomy of the radius to prevent dislocation of radial head

3. Soft tissue reconstruction

4. Vascularized fascio-fat graft interposition.

Separation of the synostosis

This is performed through a combined anterior and posterior approach. The posterior skin incision extends from the lateral epicondyle distally and medially to the posterior crest of the ulna, exposing the anconeus. The anconeus is detached from the ulna with a periosteal sleeve. The synostosis is identified. The proximal and distal borders are delineated. The distal border is usually demarcated by interosseous recurrent vessels that are constantly found at this site. A synchondrosis is usually seen between the radial head and proximal ulna. Both the synostosis and synchondrosis are separated by using a steel burr. The direction of separation is confirmed from the preoperative CT scans. After separation of the synostosis from the ulnar border of the radius, the forearm can be placed in a neutral position. The synostosis is then widely excised after detaching the biceps tendon from its anterior aspect and the radial head is trimmed to the desired shape.

The next stage of the operation is to expose the proximal forearm through a Henry approach. The fibrous extension of the biceps tendon to the proximal ulna is then removed in order to improve forearm rotation.

Radial osteotomy

A shortening angulation osteotomy either in flexion for posterior dislocation or extension for anterior dislocation is required to stabilize the dislocated radial head. This is achieved by removing a wedge of trapezoid bone between the insertion of the supinator and the pronator. The forearm is then placed at 20–30° of supination and the radial osteotomy fixed with a plate and screws.

Soft tissue reconstruction

The radial capsule and the annular ligament are reefed and the detached biceps tendon reattached to the posterior cortex of the radius to make it a strong supinator. The anconeus muscle is pulled anteriorly and attached to the brachialis tendon.

Vascularized fascio-fat graft

A vascularized fascio-fat graft based on the profunda humeri is harvested. This graft is 5–10 mm longer and 5–10 mm wider than the space created by separation of the synostosis. The graft is placed between the separated radius and ulna in an anterior-to-posterior direction and the donor vessels anastomosed to the radial recurrent vessels. Through the anterior approach an alternative vascularized distal radial artery-based fascio-fat graft can also be used without the necessity of reanastomosis, as done in one case by the senior author.

The wounds are closed and an above-elbow cast is applied with the forearm in neutral rotation and the elbow in 90° of flexion. The cast is maintained for 4 weeks (Fig. 15.1).

Figure 15.1 Operative procedure.

Rotational osteotomy

An alternative procedure to mobilize the synostosis is a rotational osteotomy.^15,16

These have been described at two sites in the diaphysis of the radius and ulna^17,18 or at one site in the distal diaphysis of the radius.^19,20 An osteotomy at the synostosis is extremely complex, whereas an osteotomy at two sites in the diaphysis of the radius and ulna is easier, with fewer reported complications.

A single osteotomy of the radius can be performed subperiosteally distal to the synostosis. This has the advantage of a single surgical scar and avoids the need for internal fixation. This osteotomy is recommended in children between the ages of 3 and 6 years and is most commonly performed.

A single osteotomy of the distal radius is performed through a 4 cm skin incision made distally on the radiovolar aspect of the distal forearm. The diaphysis of the radius is exposed and the superficial radial nerve and the radial artery are identified and retracted ulnarward. A sharp longitudinal incision is made on the periosteum and reflected atraumatically from the radius to preserve the longitudinal continuity of the periosteum. Several drill holes are made in the transverse line to the radius using a K-wire and a transverse osteotomy is performed using a chisel (Fig. 15.2A). A meticulous periosteal repair is then performed and the distal forearm maximally supinated and immobilized in an above-elbow plaster cast (Fig. 15.2B).

Figure 15.2 (A) Subperiosteal transverse osteotomy in the radial diaphysis. (B) Above-elbow cast with forearm in maximum supination.

Outcome including literature review

Operative procedures used in the treatment of congenital radio-ulnar synostosis fall into two major groups. Group 1 procedures are designed to restore rotation and remove the synostosis, while group 2 procedures restore the forearm to a fixed functional position. Several authors have reported poor results after separation of the synostosis and interposition of fat, muscle or silicone.^11,21–23

Kanaya et al performed the mobilization procedure in seven boys (average age 8 years 2 months) who had isolated proximal radio-ulnar synostosis. All seven children had a dislocation of the radial head preoperatively. A shortening-wedge osteotomy of the radius was performed in four patients at the time of the index procedure, while in three patients the radial head was manually reduced without an osteotomy. The radial head subsequently redislocated in the latter three patients and one of them had an osteotomy 2 years and 8 months after the index procedure to reduce and maintain the radial head alignment. At follow-up the total range of forearm rotation achieved averaged 71° with average supination of 26° and average pronation of 45°. There were no cases of reankylosis at an average follow-up of 4 years 4 months.¹² The group who had a radial osteotomy in addition to the index procedure had an average arc of 83° of forearm rotation compared to the second group, who had an average arc of rotation of 40°.

Kamineni et al²⁴ have described a new technique of restoring forearm rotation in post-traumatic proximal radio-ulnar synostosis. This involves resecting a 1 cm thick section of bone from the proximal radial shaft distal to the synostosis. They performed this surgery on seven patients and reported an improvement in forearm rotation from an average fixed pronation of 5° to an average arc of 98°.

Osteotomies to achieve a more functional forearm position are suitable for patients with marked pronation deformity. Green and Mital¹⁶ have reported the best results with derotational osteotomies through the fusion mass. This has the advantages of achieving a better correction and rapid healing of the osteotomy owing to the good coaptation of the divided ends. In their series results were excellent in 50%, good in 33%, fair in 6% and poor in 6%. Ogino and Hikino¹ also favoured an osteotomy through the fusion mass, but recommended resection of 0.5 cm of bone at the osteotomy site in order to shorten the forearm. All the patients in their series had complete relief of symptoms.

Goldner and Lipton recommended derotation in the distal forearm to minimize circulatory problems.²⁵ The osteotomy was done in the distal radius alone for younger patients and in both the radius and ulna in older patients.

A single osteotomy technique distal to the synostosis was reported by Kashiwa et al in 1999¹⁹ and subsequently modified by Fujimoto et al.²⁰ This osteotomy is performed at the distal radial diaphysis. Fujimoto presented the results of this osteotomy in three patients (four forearms) with congenital radio-ulnar synostosis. Bony union was achieved in all cases. The period of immobilization in plaster varied from 6 to 9 weeks. All patients achieved good correction, with functional improvement in activities of daily living.

Bolano²⁶ has shown good results with an osteotomy and gradual correction of the rotation using a ring fixator. This technique is useful as there is less risk to the neurovascular structures and it has the added advantage of allowing the patients to choose the most functional position of their forearm.

The ideal position to place the forearm after the osteotomy depends on several factors. If the synostosis is bilateral and both hands are in pronation, Simmons and Waters²⁷ recommended 20° of pronation in the dominant hand and a neutral position for the non-dominant. In unilateral cases, Green and Mital¹⁶ recommended 20–25° of supination as the supinated forearm can attain some amount of functional pronation by internal rotation, flexion and abduction at the shoulder. Ogino and Hikino¹ from Japan recommend at least 70° of palmar supination, especially in the non-dominant hand, to enable the use of chopsticks by their patients. Hence the amount of angular correction is dependent on customs, dominant hand and bilaterality.

Complications

Following excision of a synostosis and mobilization of the forearm, reankylosis can occur, giving poor results^21,22 and a risk of radial head dislocation and posterior interosseous nerve palsy.

Complications reported following derotational osteotomies include neurovascular compromise. Green and Mital reported one case of severe Volkmann’s contracture and one case of posterior interosseous nerve palsy from their series of 13 patients. Ogino and Hikino¹ reported two posterior interosseous nerve palsies in 13 operations, both of which spontaneously recovered.

Simmons and Waters²⁷ reported a 36% complication rate with loss of correction in three out of 21 children and an 18% incidence of vascular compromise. A decreased range of elbow movement has also been reported.

Conclusions and personal view