Scaphoid Nonunions: Predictive Factors

INTRODUCTION

Nonvascularized grafting for scaphoid nonunion has been undertaken for many years, and wide-ranging results have been reported for various techniques. These techniques include the use of either cancellous or corticocancellous bone taken from various sites, which generally include the distal radius or iliac crest. This grafting has been supplemented by various techniques of fixation, ranging from simple impaction of the graft, Kirschner (K) wires, to screw fixation. Additional factors that have an effect on bone healing include vascularity of the scaphoid fracture fragments, site of fracture, patient age, smoking, previous surgery, and others. The problem in determining the predictive factors regarding scaphoid union is further compounded by the fact that the techniques and criteria to assess union vary widely and follow-up is variable.

As a consequence, the number of variables affecting the chances of union is large. Various authors from around the world have researched these variables. The purpose of this chapter is to collate this evidence and allow the surgeon to make a prediction as to the likelihood of the success of surgery.

BACKGROUND

The only reliable method of achieving union is by some form of bone graft. Internal fixation alone may control symptoms for many years, but it leads to bony union in only a minority of cases.

Bone grafts may be:

1.

Longitudinal pegs, introduced along the same axis as a screw.
2.

Inlay grafts, either from the dorsum or, more commonly, from the palmar aspect as recommended by Russe.
3.

Interposition grafts, in which the un-united surfaces on each fragment are excised and either a wedge or trapezoidal-shaped graft is inserted in between. Generally, this type of graft is inserted from a volar approach because this protects any dorsal vascularity of the scaphoid but also allows correction of any flexion deformity at the nonunion site. This is probably the most widely used technique.

As to the source of graft, although various biologic substitutes are now being tried, bone graft has traditionally been taken from either the distal radius or the iliac crest. The general feeling is that the latter is superior, although no study to date has clearly shown this to be the case. Most studies using either source of bone graft show equal rates of union. Tambe and colleagues compared two cohorts of patients that were generally comparable with regard to location of fracture, duration of nonunion, and type of implants used for fixation. The union rate was not influenced by the source of bone graft used.

Methods of fixation have essentially varied between K wires and screws of various designs. The headless scaphoid screw, pioneered by Herbert, can apply compression across the nonunion site with or without bone graft and can be buried within the bone. Whether this compression enhances healing at the nonunion site remains unknown, although it undoubtedly results in a more rigid fixation. The success of the Herbert screw has resulted in many further developments in headless screws in both their compressive abilities and their ease of use, particularly for percutaneous insertion. The evidence as to whether any method of fixation is superior to another remains sketchy. However, most surgeons do tend to use some form of screw, leaving K wires for the salvage situation only. If it is technically not possible to insert a screw, then a K wire may be the only option.

Much has been written about all the other factors. Schuind and colleagues reported a multicenter study of 138 patients with scaphoid nonunion. The clinicians attempted to assess the prognostic factors for bone healing or failure within this group. They reported healing in 75% of cases. Detailed statistical analysis revealed heavy manual work, duration of nonunion of more than 5 years, associated radial styloidectomy, and duration of postoperative immobilization as being associated with a significantly decreased likelihood of healing. Multivariate analysis, however, identified only the delay between the initial trauma and the definitive treatment as being significant; specifically, a duration of 5 years or more has a lower rate of union.

Avascular necrosis, particularly of the proximal pole, is generally held to be a poor prognostic factor. Although the diagnosis can sometimes be contentious, there is no doubt that union can be very difficult to obtain.

Not surprisingly, the reported union rates vary widely. Some authors report union rates of 100%, whereas others report a value between 75% and 80%. Systematic review of 147 publications including 5246 cases of scaphoid nonunion by Munk and Larsen revealed a union rate of 80% achieved with nonvascularized bone grafting without internal fixation. The figure with internal fixation was 84%.

The final difficulty is twofold, regarding both the assessment of union itself and after what time period it can be stated that bone-grafting surgery has failed. For the latter, there is no clear definition; however, most authors give this type of surgery a minimum of 12 months before accepting that union is no longer likely to occur. With regard to assessing union itself many publications have used criteria described by Dias in 2001. These include the presence of trabeculae crossing the fracture site or, conversely, the presence of sclerosis at the fracture edges. Undoubtedly, these criteria help surgeons to quantify union, but they are open to a degree of interpretation. There is no doubt that magnetic resonance imaging (MRI) or computed tomography (CT) scans are more accurate than plain radiography. However, they are more expensive and time-consuming, and they may not be readily available.

PREDICTION OF SUCCESSFUL SURGERY

In an attempt to understand the place of all variables in the likely success of simple nonvascularized bone grafting and internal fixation, we undertook a retrospective review of case records. This review identified 159 patients in whom an established nonunion of the scaphoid had been treated by bone grafting and internal fixation. There were 107 patients treated at Wrightington Hospital and 52 patients at Arrowe Park Hospital between April 1991 and February 2003 and between April 1996 and November 2002, respectively. Nonunion was defined as persistence of a fracture gap for at least 3 months after the initial trauma, with associated resorption of bone and cystic changes at the fracture site, as seen on radiographs. There were five patterns of presentation ( Table 19-1 ).

TABLE 19-1

Pattern of Presentation of Nonunion of the Scaphoid

Patient Group	Pattern of Presentation	No. of Patients
1	Patients with a scaphoid fracture treated in a cast and followed up adequately	37
2	Patients with a scaphoid fracture treated in a cast and followed up inadequately	15
3	Patients with a scaphoid fracture and associated perilunate dislocation or fracture of the distal radius, with immediate surgical fixation	13
4	Patients who were never treated at the time of initial injury but presented with persisting symptoms following re-injury	88
5	Patients who had previous unsuccessful surgery for nonunion	6

Patients included in the study had established nonunion of the scaphoid and had undergone nonvascularized bone grafting and internal fixation, either with a screw or K wires. A total of 19 patients could not remember the month and year of the initial injury; 9 patients had less than 6 months of follow-up, and 6 had undergone previous failed surgery for established nonunion. These 19 patients were thus excluded from the study, as was one patient with a fracture in the coronal plane, leaving 124 patients (126 nonunions) in the study ( Table 19-2 ). All operations were performed by the senior authors. A dorsal approach was used in 19 patients with fractures involving the proximal pole. In the remainder, the fracture was approached through an anterior incision. Fibrous tissue between the fragments was excised, and the fracture surfaces were curetted. If a dorsal intercalated segment instability pattern was present, a K wire was used to correct the deformity. The bone graft was obtained from the iliac crest in 70 patients and from the distal radius in 54. Corticocancellous bone grafts were used when the bone defect was large, and multiple cancellous bone chips when the defect was small. Internal fixation was accomplished using two 1.6-mm K wires in 46 nonunions and a single screw in 80 cases. Herbert (Zimmer, Warsaw, Indiana), 3.5-mm AO (Synthes, Waldenburg, Switzerland), and Acutrak screws (Acumed, Beaverton, Oregon) were used for fixation in 57, 15, and 8 cases, respectively. The wrist was immobilized for 8 to 12 weeks after operation. All patients were followed up clinically and radiologically by one of the two operating surgeons at 6 and 12 weeks, and at 6 months or until union was achieved.

TABLE 19-2

Demographic Details (Based on Number of Nonunions)

Mean age in years (range)	28 (9−59)
Gender
Male Female	119 7
Hand dominance
Dominant Nondominant	62 64
Mean time interval from injury to surgery, years (range)	4.5 (0.25−16)
DISI (radiolunate angle >10°)	56
Osteoarthritis
Grade 1 Grade 2	21 4
Fracture pattern
Transverse Vertically oblique Horizontally oblique	103 11 12
Displacement at fracture site
Displaced (>2 mm) Undisplaced	93 33

DISI, dorsal intercalated segment instability.

The fracture site was described by the fragment ratio, measured using plain radiographs. These radiographs were converted to digital images using a Nikon Coolpix 5000 digital camera (Nikon Corp., Tokyo, Japan) with standard settings. In the digital format, it was easier to identify the margins of the fragments and to make accurate measurements of their size. Since no single view enables visualization of the fracture site in all cases, the long axis of the scaphoid and the fracture are best visualized in the ulnar-deviated and posteroanterior semipronated oblique views. To calculate the amount of projectional error causing variation in measurement between these two views, the whole length of the scaphoid was measured in 138 films of normal scaphoids, taken consecutively, with standard magnification settings. The estimated standard deviation (SD) within subjects was 0.58 mm, and the intraclass correlation coefficient was 0.932. The repeatability index of 1.6 mm is equivalent to 6.7% of the mean length of the scaphoid. Because the variation in the measured length was not significant, these two views were used to calculate the fragment ratio. The ulnar-deviated posteroanterior view was used in 67 nonunions and the semipronated oblique view in 59 nonunions.

The bone fragments were measured using the measurement tool in Adobe Photoshop version 6.0 (Adobe Systems, Inc., San Jose, California) ( Fig. 19-1 A and B ). Horizontal lines were drawn at the ends of each fragment to define the length of the fragment. The middle of the fragment was identified, and a line was drawn between these points to connect the horizontal lines. The length of this line was measured to determine the size of the fragment. The fragment ratio was then calculated by dividing the proximal fragment size by the sum of the sizes of the proximal and distal fragments. This value was used to describe the site of nonunion. In the 11 vertically oblique fractures, the same procedure was performed, but because the line is drawn along the middle of a fragment, it measured the mean length of both.