16 Absorbable Scaphoid Screws
▪ Rationale and Basic Science Pertinent to the Procedure
A variety of implants have been used for the fixation of scaphoid fractures.1 The use of absorbable screws for this purpose has recently been studied.2 – 5 Absorbable bone fixation devices have some theoretical advantages in the treatment of cancellous bone fractures compared with metal devices. One advantage is that removal of the implant is not required after bony union has occurred.6 Another advantage is the ability to perform a magnetic resonance imaging (MRI) evaluation on the operated wrist. In contrast to metal implants, absorbable materials do not produce an artifact on MRI.7 The healing of an osteonecrotic proximal pole of the scaphoid after surgery can thus be followed using MRI when an absorbable screw is used. It has even been proposed, based on experimental data, that MRI might detect an impending nonunion earlier by providing detailed information about the early fusion process because computed tomography (CT) visualizes only mineralized bone.8 Studies show that an MRI study can be used after implantation of a titanium alloy, but some artifacts are still likely to occur.9 Absorbable materials have a lower elastic modulus, which may prevent the stress shielding seen following internal fixation with a metallic implant.10 , 11 The current generation of self-reinforced absorbable poly-L-lactic acid (SR-PLLA) implants seem to retain their mechanical strength long enough for the scaphoid to heal10 , 12 and have fewer side effects than earlier absorbable implants.13
The indications for scaphoid screw fixation include any acute displaced scaphoid fracture as well as undisplaced fibrous nonunions. It is our preference to use an absorb-able scaphoid screw mostly for the rare case of scaphoid nonunion in a growing child, where a rigid metal implant might affect the growth of the bone. They can be used in any patient with a scaphoid nonunion in lieu of a metallic screw, especially in those cases where sequential MRIs are used to assess the healing of an osteonecrotic proximal scaphoid pole after surgery. Although we have used absorbable screws through an open approach the recent development of cannulated absorbable compression screws allows the use of percutaneous techniques.
Contraindications to the use of a bioabsorbable screw parallel those of metallic screws and would include infection, poor bone stock, and a very small proximal or distal pole that would not allow adequate purchase.
▪ Surgical Technique
The operation is performed under general anesthesia with the patient in a supine position. A tourniquet is used in all cases. A 4 to 5 cm longitudinal incision is made along the radial border of the flexor carpi radialis tendon centered over the tip of the radial styloid. Dissection is performed between the flexor carpi radialis tendon and the radial artery down to the joint capsule. The superficial communication branch of the radial artery is ligated and divided. The joint capsule is divided longitudinally over the scaphoid, and the fracture is exposed. In the case of a nonunion, the sclerotic surfaces of both fragments are removed with a chisel or with an oscillating saw. A tricortical corticocancellous bone graft is obtained from the iliac crest and shaped to restore the normal contour of the scaphoid bone when interposed between the fragments. Because the screw that we use is full-threaded and by itself does not provide any compression, the fragments are compressed with a reduction clamp during fixation to ensure tight bony contact. A temporary K-wire is inserted at this point to hold the reduction, and the screw hole is drilled parallel to the K-wire under fluoroscopic control to the desired depth and direction. A depth gauge is used to measure the screw length, then the drill hole is tapped and countersunk to bury the screw head. Irrigating the drill hole with saline is advised to facilitate screw insertion because excess friction could result in screw breakage. Definitive fixation is performed using either a 2.0 or 2.7 mm absorbable SR-PLLA screw (Bionx Implants Ltd., Tampere, Finland) ( Fig. 16.1 ). The screw head is buried in the bone. Alternatively the screw head can be cut flush with the bone using high-temperature loop tip cautery.14 Any temporary K-wires are removed at this point. If satisfactory fixation is not achieved with one screw, then another absorbable screw is applied. If there is no space for another screw a supplementary stainless steel compression fixation pin (Orthofix Fragment Fixation System, Orthofix Srl., Bussolengo, Italy) can be used to strengthen the fixation. The volar radiocarpal ligaments and joint capsule are then repaired.
Postoperative protocol includes immobilization of the wrist in a short-arm/thumb spica cast for an average of 8 weeks (4 to 9 weeks), depending upon radiographic evidence of bony union. Follow-up radiographs are taken in 1-month intervals until solid union is achieved. Although the absorbable screws are not visible on the radiographs, it is possible to visualize the bony canal, which creates a silhouette of the implant.