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Scaphoid Nonunions.

Scaphoid nonunions are complex injuries that require a thorough understanding of the natural history and treatment options to provide optimal patient results. Prompt diagnosis and treatment of scaphoid fractures often leads to bony union; however, nonunions are not uncommon given the tenuous blood supply of this region. When a scaphoid nonunion occurs, prompt treatment decreases the risk of wrist collapse and future arthritis. Surgical management depends on the location of the nonunion and the vascular status of the proximal fragment. Salvage operations are reserved for advanced stage disease seen in chronic scaphoid nonunions.

Anatomy

The word scaphoid is derived from the Greek work scaphoides which means "boat-shaped." This accurately describes the shape of this carpal bone, which includes articular cartilage covering the majority of its surface. The scaphoid is divided into three zones: the distal pole, the waist, and the proximal pole. The scaphoid's tenuous blood supply is well studied and relevant to why nonunions develop. The dorsal branch of the radial artery, which is the scaphoid's main blood supply, enters through the dorsal ridge and supplies 80% of the scaphoid in a retrograde manner. The volar arterial branch supplies the distal scaphoid and accounts for the remaining 20% of the blood supply. The proximal scaphoid relies entirely on intramedullary blood flow, which is often disrupted in scaphoid fractures. (1)

Evolution from Scaphoid Fracture to Nonunion

Scaphoid nonunions are largely preventable; timely recognition and acutely treated scaphoid fractures have a union rate of 95%. (2) Misdiagnosis of scaphoid fractures as simple wrist sprains leads to delay in appropriate treatment. This is often due to difficulty in identifying scaphoid fractures on plain radiographs; it has been reported that 16% of scaphoid fractures are not detected on plain radiographs. (3) The most common scaphoid fracture pattern is oblique, and these fractures can be undetected unless the x-ray beam lies in the same plane as the fracture. Missed fractures are not often immobilized, leading to motion at the fracture site and increased nonunion rates. Multiple x-ray views are required to fully evaluate the complex geometry of the scaphoid including a posteroanterioa (PA), lateral, PA ulnar deviation, and navicular views.

If a scaphoid fracture is suspected but x-rays are negative, further intervention is needed. The first option is to immobilize the patient and repeat x-rays in 10 to 14 days. Resorptive changes at the fracture site may make the fracture more evident at that time point. Recent studies have shown this to have poor interobserver reliability, and many clinicians now recommend obtaining advanced imaging. An acute magnetic resonance image (MRI) has a sensitivity of a 100%, a specificity of 95% to 100%, a high interobserver reliability, and is also able to diagnose soft tissue injuries in proximity to the suspected fracture. Dorsay et al. (4) conducted a cost effectiveness analysis and found evaluation with a MRI to cost $770 versus $677 for the traditional follow-up of splinting and repeat x-rays. Given the overall productivity loss due to missed work, immobilization, and the need for an MRI if a definitive diagnosis was unable to be made, they recommended obtaining an immediate MRI.

Nonunion Defnition and Classification

Scaphoid nonunion terminology with regard to chronicity is important to defne since it leads to alterations in treatment decisions. A union is defined as trabeculae crossing the fracture site on standard radiographs in three or four views. The average time for scaphoid union is 3 to 6 months. A delayed union is defined as incomplete osseous union at 4 months. A nonunion is an arrest in the fracture repair process with lost potential for healing at 6 months. Radiographically, there is persistent lucency along fracture lines with smooth rounded sclerotic margins. Nonunions occur in 5% to 10% of all scaphoid fractures. (2,5,6)

Multiple classifications were developed to try to predict which types of scaphoid fractures would fail to unite. Russe developed a classification based on fracture line orientation. (7) Herbert developed a classification system based on location and fracture pattern. (8) However, these classifications have been found to be cumbersome with only fair interobserver and intraobserver reliability and low predictability of fracture union. (9) The most commonly used classification system is based on the location of the fracture and consists of scaphoid tubercle fractures, scaphoid waist fractures, and proximal pole fractures. The classification system has been found to predict avascular necrosis (AVN) rate and nonunion rate. For instance, the proximal one-fifth has a reported scaphoid AVN rate of 100% while the proximal one-third of the scaphoid has an AVN rate of 33%. (10,11)

The risk of nonunion is also affected by fracture characteristics that lead to instability and motion at the fracture site. These characteristics include greater than 1 mm of displacement, a lateral intrascaphoid angle greater than 35[degrees], bone loss or comminution, perilunate fracture-dislocation, dorsal intercalated segmental instability alignment, and proximal pole fractures. In addition, smoking history also increases the risk of nonunion. (1)

Nonunion Characteristics

There are two different patterns of scaphoid nonunion displacement: volar and dorsal. The location of the nonunion line relative to the dorsal apex ridge of the scaphoid determines the pattern of the nonunion. Dorsal displacement is seen in proximal scaphoid nonunions, while volar displacement is most often seen with distal scaphoid nonunions. When a scaphoid nonunion is present, there is the potential for independent movement of the lunate and its attachment to the proximal pole of the scaphoid from the distal scaphoid fragment. (12) Because of the extension force from the triquetrum transmitted through the lunotriquetral ligament, the lunate extends the proximal pole through the scapholunate ligament and the distal pole fexes producing a "humpback deformity." Volar flexion of the scaphoid and dorsiflexion of the lunate can result in dorsal intercalated segmental instability (DISI) deformity. This dynamic deformity alters the anatomy and kinematics of the joint and causes progression to a scaphoid nonunion advanced collapse (SNAC) if not treated. (13,14)

Imaging of Scaphoid Nonunions

Full visualization of scaphoid nonunions can be difficult due to the complex geometry of this carpal bone. As a result, multiple views and modalities are required. Plain radiographs are similar to those of a scaphoid fracture and should include a PA, lateral, 45[degrees] pronated and supinated oblique view, and a scaphoid view. Radiographic evidence of nonunion includes widening of the fracture site, cleft and cyst formation, sclerosis, and smoothing of the fracture edges. In cases of chronic scaphoid nonunions, radiographs should be used to evaluate the presence of osteonecrosis, carpal collapse, carpal instability, and degenerative change. Specifically, the radioscaphoid and capitolunate joints should be evaluated for scaphoid nonunion advanced collapse, as this pattern of chronic injury and arthrosis dictates treatment options. A scaphoid fracture is considered healed if there is no pain at the fracture site and plain radiographs show crossing trabeculae. Plain radiographs alone may be unreliable in confrming fracture union during the first 4 months from injury. (15) It is, therefore, recommended that if scaphoid nonunions are suspected within 6 months from the injury, other advanced imaging modalities such as computed tomography (CT) or magnetic resonance imaging (MRI) should be utilized. (16)

A CT scan is an excellent three-dimensional study to evaluate scaphoid nonunions. During imaging, the CT scan should proceed along the longitudinal axis of the scaphoid. Computed tomography evaluates bony structures better than MRI, which is helpful in diagnosing the three-dimensional location of a nonunion and preoperative planning of any bony deffect that may require grafting at the time of surgery.

More importantly, CT imaging allows for evaluation for scaphoid collapse that develops when a nonunion is distal to the apex of the dorsal scaphoid ridge. This is measured either using the lateral intrascaphoid angle or a scaphoid height to length ratio. The intrascaphoid angle is measured by the intersection of two lines drawn perpendicular to the diameters of the proximal and distal poles on the sagittal view. Values over 35[degrees] indicate collapse. The height to length ratio is another measurement used to evaluate collapse and is measured by dividing the height of the scaphoid by the length.

Magnetic resonance imaging is the modality of choice to evaluate the vascular status of the scaphoid's proximal pole. Avascular necrosis manifests as low signal on both T1 and T2 weighted images. However, the benefits of predicting AV N by MRI are surprisingly inconsistent. When preoperative MRI findings are compared with intraoperative observation of proximal pole vascularity and histologic specimens, MRI has low predictive value and an accuracy of only 59%. (17)

Surgical Management

Mack et al. (14) found a direct correlation between the duration of a nonunion DISI deformity and the progression of arthritis. Arthritic changes were present in 97% of patients with nonunions assessed 5 years after injury, and the degree of arthritis was directly proportional to duration of the nonunion. Conversely, outcomes of patients who had a scaphoid fracture who went on to union developed arthritis only 2% of the time at 30-year follow-up. (18) This leads to a clear treatment objective, which is to appropriately treat all scaphoid fractures to optimize the potential for union through casting or operative fixation, and if a nonunion develops, prompt treatment should ensue.

Surgical intervention typically occurs at 6 months or if there is no progression in healing, allowing adequate time for a potential primary union while preventing future carpal instability and arthritis. The goals of surgery are to restore bone loss, carpal collapse, humpback deformity, vascularity, and promote union with the ultimate goal of prevention of arthritis. (19)

Like most complex pathologies, it is important to have a systematic approach to treatment. Traditionally, the first step is to determine the location of the nonunion. This is important because scaphoid waist nonunions may be approached volarly or dorsally depending on the vascular status of the proximal pole. Proximal pole nonunions are better visualized when approached dorsally regardless of the vascular status. While this has been the traditional treatment algorithm, recent data has lead to a shift in this algorithm. (5)

The volar approach is utilized when a correction of a humpback deformity is needed. Other advantages include easy access for retrograde internal fixation and preservation of the dorsal blood supply. It is not recommended for nonunions of the proximal pole due to the difficulty in access but gives excellent exposure of waist nonunions. A hockey stick incision is made along the course of the flexor carpi radialis (FCR) and extended distally along the border of the glabrous skin of the thenar eminence. Dissection continues between the interval of the FCR and the deep branch of the radial artery. The volar carpal ligaments are incised perpendicular to their fibers to expose the distal pole and waist of the scaphoid. Care should be taken to protect or repair the radioscaphocapitate ligament if transected in the approach since this ligament prevents the proximal pole from translating volarly. (7) If a humpback deformity is present, bone graft may be placed to restore structural alignment.

The dorsal approach is used for proximal pole nonunions and AVN. The blood supply enters at the dorsal ridge and preserving the tenuous dorsal branch of the radial artery is essential. Benefts of this approach include increased control of the small proximal fragment, which is often difficult to target from the volar approach, a decreased risk of proximal pole displacement, and access to dorsal vessels that are optimal for vascular grafts. The dorsal approach begins with a longitudinal incision centered over the radiocarpal joint. The sheath of the extensor pollicis longus muscle (EPL) is released. The extensor digitorum communis (EDC) tendons are retracted ulnarly and the extensor carpi radialis brevis muscle (ECRB), extensor carpi radialis muscle (ECRL), and EPL are retracted radially exposing the underlying joint capsule. The radiocarpal joint capsule is then incised to expose the scapholunate articulation.

After determining the location of the nonunion, the vas-cularity of the proximal pole and the need for bone graft is determined. There are two types of bone grafts that are considered for scaphoid nonunions: non-vascularized and vascularized bone grafts. Non-vascularized bone grafts were the first form of bone grafting, which have limited osteogenic potential. These grafts heal by creeping substitution in which the graft is replaced with live bone by osteoclastic resorption and new bone formation by osteoblasts. This does cause significant mechanical weakening of the graft during the incorporation process. Historically, union rates have been reported from 60% to 90%, but drop to 40% to 67% when proximal pole AVN is present. (20-22)

Non-vascularized bone graft is categorized in two subdivisions: corticocancellous and cancellous. Corticocancellous bone has superior mechanical properties due to the inclusion of cortical bone. It is able to fll structural deffects, restore scaphoid length, and correct carpal collapse. For this reason, it is well suited to correct humpback deformities. Examples of this graft include iliac crest and distal radius graft. Cancellous bone lacks the mechanical strength of corticocancellous graft so it is unable to correct structural deffects as effectively. Cancellous graft is most commonly harvested from the iliac crest and distal radius. (23) Studies have evaluated the results of corticocancellous and cancellous bone for scaphoid nonunions with a vascular proximal pole. Union rates of 92% to 95% have been reported with statistically significant improvements in time to union and wrist flexion for cancellous grafts. The corticocancellous grafts did demonstrate improved Mayo wrist scores and improved intrascaphoid angles. (23) A randomized controlled trial was conducted to evaluate which of the corticocancellous grafts provide superior outcomes. Results showed no difference in wrist motion, functionality, and union rates. Distal radius grafts are recommended as they are less invasive than iliac crest grafts. Iliac crest grafts are associated with postoperative pain, scarring, and hernias. (24) Recent studies have shown that distal radius cancellous autograft with screw fixation yielded a union rate of 100% and were able to correct the intrascaphoid angle from 49[degrees] to 32[degrees]. Additionally, grip strength increased to 103% compared to the contralateral side. (25)

Non-vascularized bone graft is a successful option for scaphoid nonunions with a vascularized proximal pole; however, other options may be needed for patients demonstrating evidence of AVN. (20-22) Vascularized corticocanellous bone grafts provide structural support and provide a vascular supply to the nonunion site to allow for healing. Vascularized graft donor locations include the distal radius, medial femoral condyle, metacarpal, and ulna. These technically challenging procedures can produce union rates as high as 100%. (26-28)

The 1,2 intercompartmental supraretinacular artery (1,2 ICSRA) pedicle graft is a commonly used vascularized graft. It originates from the radial artery 5 cm proximal to the radiocarpal joint. (29) It is most successfully used for AV N of the proximal pole in cases that lack collapse, humpback deformity, and carpal instability. (21) Early outcomes with 1,2 ICSRA were promising, and a 100% union rate has been reported. (30) Meta-analyses of early results demonstrated a union rate of 88% for proximal pole AV N using the 1,2 ICSRA. (30) Long-term follow-up studies, however, have been less enthusiastic with union rates approaching 50% and high rates of failure when humpback deformities, collapse, or carpal instability are present. (21)

Because of the inconsistent findings, a randomized trial was conducted between non-vascularized iliac crest graft and vascularized 1,2 ICSRA graft for the treatment of proximal pole nonunions with AVN. There was a 100% union rate with non-vascularized graft and a 92% union rate in the vascularized group. The investigators concluded the non-vascularized group had higher union rates because of the technical ease of the procedure, lack of pedicle to damage, and increased stability of fixation. Non-vascularized grafts were secured with Herbert screws while vascularized grafts were secured with k-wires. Additionally, non-vascularized grafted patients underwent a volar approach that allowed for correction of humpback and DISI deformities, while these deformities were not able to be corrected dorsally in the vascularized bone graft patients. (31)

Alternatively, vascularized medial femoral condyle autograft has demonstrated promising results. It is harvested from the ipsilateral medial femoral condyle, which produces an osteochondral autograft with a convex surface that closely resembles the curvature of the proximal scaphoid. The vascular pedicle originates from the descending geniculate artery. Multiple studies report union rates of 100% using this technique. (32-34) One prospective randomized trial compared medial femoral condyle graft and 1,2 ISRCA; this study demonstrated union rates of 100% and 40%, respectively. The medial femoral condyle vascularized autograft results are promising; however, it requires a separate surgical approach and can cause complications such as donor site pain and femur fracture. (32-34)

Scaphoid Nonunion Advanced Collapse (SNAC)

Chronic scaphoid nonunions lead to SNAC wrist, which requires a salvage procedure to relieve pain and maximize function for the patient. The treatment options are based on the stage of the scaphoid nonunion advanced collapse. Stage I is defined as arthrosis of the radial styloid and the procedure of choice is a radial styloidectomy with nonunion repair. Stage II is defined as advancing arthrosis to the proximal scaphoid and the treatment options include a four-corner fusion or a proximal row carpectomy (PRC). Stage III is classifed as advancement of the arthrosis to the capitolunate joint, which is treated with a four-corner fusion or total wrist fusion. (35-38)

The treatment for stage II is controversial as there are advantages and disadvantages of the four-corner fusion and the PRC. Advocates of the four-corner fusion cite early studies that described greater grip strength and a lower chance of progression of radiocarpal arthritis requiring an eventual wrist fusion. Advocates of PRC cite an easier technique, earlier range of motion postoperatively, and the decreased risk of complications due to nonunion or hardware failure. Removal of the proximal carpal row leads to articulation of the capitate with the lunate fossa of the distal radius resulting in a mismatch of curvature between the shape of the capitate and lunate fossa. Opponents of PRC cite this mismatch as a theoretical reason for more rapid progression of radiocarpal arthritis; however, the original PRC description warned against performing the procedure if degeneration of the capitate was present since the capitate will articulate with the lunate fossa, potentially causing pain and future conversion to a wrist fusion. (35-37)

A meta-analysis was performed to compare outcomes of PRC and four-corner fusion. This analysis found similar pain relief, discomfort with end range of motion, grip strength, and conversion to total wrist fusions between the two procedures. Proximal row carpectomy had a slightly increased range of motion (10[degrees]) and fewer complications, but a higher potential for conversion to wrist fusion in younger active patients. Current recommendations are for four-corner fusion in patients under 35 years old and in high-demand patients in their 40s and 50s. Proximal row carpectomy should be reserved for older, less active patients. (36)

Conclusion

Scaphoid nonunions are a preventable pathology with heightened clinical suspicion, appropriate imaging, and acute treatment of scaphoid fractures. Acute management of a scaphoid nonunion is vital to decrease the risk of arthritis. The surgical approach and graft options depend on the location of the nonunion and the vascular status of the proximal pole. The optimal treatment algorithm continues to be debated. In the case of chronic scaphoid nonunions with advanced degenerative change, salvage procedures may be necessary to improve pain and function in this patient subset.

Disclosure Statement

None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony.

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Christopher S. Klifto, MD, Austin J. Ramme, MD, PhD, Anthony Sapienza, MD, and Nader Paksima, DO

Christopher S. Klifto, MD, Department of Orthopedic Surgery, Duke University Medical Center, Durham, North Carolina, USA. Austin J. Ramme, MD, PhD, Anthony Sapienza, MD, and Nader Paksima, DO, Department of Orthopedic Surgery, NYU Langone Orthopedic Hospital, NYU Langone Health, New York, New York, USA.

Correspondence: Christopher Klifto, MD, Department of Orthopedic Surgery, NYU Langone Orthopedic Hospital, NYU Langone Health, 301 East 17th Street, New York, New York 10003, USA; cklifto@gmail.com.

Klifto CS, Ramme AJ, Sapienza A, Paksima N. Scaphoid nonunions. Bull Hosp Jt Dis. 2018;76(1):27-32.

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Author:Klifto, Christopher S.; Ramme, Austin J.; Sapienza, Anthony; Paksima, Nader
Publication:Bulletin of the NYU Hospital for Joint Diseases
Article Type:Disease/Disorder overview
Date:Jan 1, 2018
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