An Anatomical Database of Carpal Bone Measurements for Intercarpal Arthrodesis

INTRODUCTION

Limited intercarpal fusions are becoming increasingly popular for the treatment of degenerative arthritis, intercarpal ligament injuries, rotary subluxation of the scaphoid, midcarpal instability, scaphoid nonunion and Keinböck’s disease (Watson et al., 1999).

In some procedures, such as scaphocapitate (Fig 1), lunotriquetral and lunocapitate fusions (Fig 2), interfragmentary screws or staples are used. However, in others where maintenance of bone length is essential, such as scaphotrapeziotrapezoid (STT or “triscaphe”) fusions, Kirschner wires are the fixation method of choice (Viegas et al., 1990; Watson et al., 1999). More recently plates have been specifically developed for midcarpal fusion, though this fusion is most commonly performed using screws or staples (Fig 3). In all procedures in which screw or staple fixation is required, it is imperative to avoid overpenetration of the fixation device. Unfortunately it is sometimes difficult intra-operatively to interpret wrist radiographic images, either on plain films or when using an image intensifier, particularly in the lateral view (Figs 1b and 3b) (Compson and Heatley, 1993).

We believe it is useful to have a knowledge of the minimum depth, width and length of the relevant carpal bones as this should help avoid inadvertent overpenetration of the fixation device. Previous databases on carpal bone dimensions (Feipel et al., 1998, 1999; Patterson et al., 1995; Schuind et al., 1992), are not applicable for this purpose because they are neither anatomical nor specific to intercarpal arthrodesis.

MATERIALS AND METHODS

Complete sets of 100 adult wrist skeletons from the Anatomy Department at Guy’s Hospital, London, were used for this study. No donor information was available to determine the demographics for each wrist. None of the specimens was paired. There were 51 left and 49 right wrists.

Nine measurements were carried out on each set, using callipers accurate to 0.1 mm (Fig 4): (A) from the radial border of the trapezium to the ulnar border of the trapezoid; (B) from the radial border of the waist of the scaphoid to the midpoint on the ulnar border of the capitate; (C) from the proximal pole of the lunate to the distal border of the capitate; (D) from the proximal pole of the triquetrum to the distal surface of the hamate; (E) the anteroposterior depth of the capitate; (F) the anteroposterior depth of the scaphoid; (G) the anteroposterior depth of the lunate; (H) the anteroposterior depth of the triquetrum; (I) the anteroposterior depth of the hamate.

Each measurement was entered into a database and the data were analysed for distribution. The mean, range and 95% confidence intervals (1.96% standard deviations on either side of the mean) for each measurement were calculated.

RESULTS

Each of our nine carpal measurements conformed to a normal distribution. Table 1 shows the results for measurements along the axes of screw fixation (Fig 4). Table 2 shows the results for measurements of the anteroposterior depths (Fig 5) of the carpal bones.

DISCUSSION

The skeletal measurements in this study provide an accurate database of measurements relevant to limited intercarpal arthrodesis procedures. The results allow estimation of maximum staple size and/or screw length preoperatively with 95% confidence intervals. This enables the surgeon to place screws and staples without the risk of breaching the distal cortex as long as they are placed centrally in the relevant bones. The maximum recommended screw length for trapeziotrapezoid fusion is 16 mm, for scaphocapitate fusion is 19 mm, for lunocapitate fusion is 25 mm and for triquetralhamate fusion is 18 mm.

Staples with no more than 9 mm depth are recommended for scaphocapitate, 10 mm for lunotriquetral and capitohamate and 11 mm for lunocapitate fusions. These staple depths should prevent inadvertent breaching of the palmar cortex in more than 95% of cases. These recommended lengths would also apply to the 2.5% of wrists, which are larger and beyond the upper limit of our 95% confidence limits. It is our opinion that patients with smaller wrists could be identified pre-operatively from clinical examination.

Previous studies show no difference between left and right wrist dimensions (Patterson et al., 1995), or wrists from different ethnic origins (Widegrow et al., 1996). However, there are gender and age differences (Patterson et al, 1995; Schuind et al., 1992). Our study is limited in that we cannot claim the sample to be representative of any particular population group. However, this can be viewed as an advantage, as despite not belonging to one particular group, the results of our study are normally distributed.

Another limitation of our study is the lack of data relating to the exact positioning of screws when these are used for fusions. We have defined an anatomical constant for measuring, but each surgeon may have a slightly different approach, insertion point or angle for placing the screw. We have used the centre points of each bone, which are all easily accessible surgically.