Lateral lunate morphology: an x-ray study

Sex-Specific Size Analysis of Carpal Bones: Implications for Orthopedic Biomedical Device Design and Therapy Planning

Consideration of the individual carpal bone characteristics of the wrist plays a key role in well-functioning biomedical devices and successful surgical procedures. Although geometric differences and individual bone sizes have been analyzed in the literature, detailed morphologic descriptions and correlations covering the entire wrist reported in a clinical context are lacking. This study aimed to perform a comprehensive and automatic analysis of the wrist morphology using the freely available "Open Source Carpal Database" (OSCD). We quantified the size of each of the individual carpal bones and their combination. These sizes were extracted in n = 117 datasets of the wrist of the OSCD in anatomical directions and analyzed using descriptive statics and correlation analysis to investigate the morphological characteristics under sex-specific aspects and to provide regression plots and equations to predict individual carpal bone sizes from the proximal and distal row dimensions. The correlations in the proximal row were higher compared to the distal row. We established comprehensive size correlations and size rations and found that there exist statistical differences between sex, particularly of the scaphoid. The regression plots and equations we provided will assist surgeons in a more accurate preoperative morphological evaluation for therapy planning and may be used for future anatomically inspired orthopedic biomedical device designs.

The three-dimensional shape symmetry of the lunate and its implications

We studied the three-dimensional (3-D) shape variations and symmetry of the lunate to evaluate whether a contralateral shape-based approach to design patient-specific implants for treatment of Kienböck's disease is accurate. A 3-D statistical shape model of the lunate was built using the computed tomography scans of 54 lunate pairs and shape symmetry was evaluated based on an intraclass correlation analysis. The lunate shape was not bilaterally symmetrical in (1) the angle scaphoid surface - radius-ulna surface, (2) the dorsal side and the length of the side adjacent to the triquetrum, (3) the orientation of the volar surface, (4) the width of the side adjacent to the scaphoid, (5) the skewness in the coronal plane and (6) the curvature of bone articulating with the hamate and capitate. These findings suggest that using the contralateral lunate to design patient-specific lunate implants may not be as accurate as it is intended.

The Effect of Lunate Morphology in Carpal Disorders: Review of the Literature

Variation in lunate morphology can exist based on the absence (Type I) or presence (Type II) of medial facet on the distal articular surface of the lunate that contacts the proximal pole of the hamate. This additional lunatohamate articulation can affect load transmission across the radiocarpal joint and exert an influence on carpal kinematics. A Type II lunate is protective against carpal instability patterns associated with scaphoid nonunions and scapholunate dissociations. It may also play a role in the progression of carpal collapse that occurs in Kienböck disease. This review summarizes the effect of lunate morphology in the outcomes of non-operative and operative treatment of carpal disorders.

Association between lunate morphology and carpal collapse patterns in scaphoid nonunions

PURPOSE

Type I lunates have a single distal facet for articulation with the capitate; type II lunates have an additional (medial) hamate facet on the distal articular surface. We retrospectively reviewed a series of patients with scaphoid nonunions to determine if there was an association between lunate morphology and the degree of carpal instability observed. Association between lunate morphology and the location of the scaphoid fracture (proximal or waist) was also investigated.

METHODS

Radiographs were evaluated for 45 patients with established scaphoid nonunions. Lunate morphology, scaphoid fracture location, and radiolunate angle were determined.

RESULTS

Type I lunates were present in 21 patients. Of these, 15 were found to have a dorsal intercalated segment instability pattern (radiolunate angle greater than 15 degrees ). By contrast, only 4 of the patients with type II lunates exhibited this pattern of instability. No significant association was found between lunate morphology and the scaphoid fracture location.

CONCLUSIONS

Type II lunate morphology is associated with significantly decreased incidence of dorsal intercalated segment instability (DISI) deformity in cases of established scaphoid nonunion (p = .0002). Lunate morphology, however, was not significantly associated with the location of the scaphoid fracture in these cases (p = .19).

Modélisation du carpe osseux et biomécanique

Carpal morphology and orientation of carpal bones are usually studied on two-plane radiography. Those measurements depend on the incidence of X-ray and on the expertise of physician. A method that eliminates both should improve the accuracy of those measurements. The digital data from computed tomography scans can be use to describe carpal geometry. We defined biometric and angular parameters allowing the study of carpal morphology and bones orientation. From digital data from computed tomography scans software can obtain bone volume, inertia principal axis and volume of ellipsoid of inertia. Bone centroid location and principal axis orientation can be used to study bones orientation. 3D distances ratio between geometry centroid of carpal bones. The measurements allowed by this methodology are numerous. A study of a more consistent series of normal wrists will allow in the future for each quantitative parameter to define the normal range. A comparative study of normal wrists and pathology wrists should allow defining, for each pathology, the most judicious quantitative parameters.

Factors affecting the sagittal alignment of the lunate

The lunate is described as having a tendency to rotate dorsally as a result of its wedge shape, with the apex towards the dorsum, but maintains an attitude of flexion in most individuals. The present study comprised CT scans of the wrist of 70 healthy volunteers. Sagittal measurements were drawn for the midcarpal and radiocarpal axes, alignment and various shape patterns of the lunate. The midcarpal axis was found to be dorsal, volar and collinear to the radiocarpal axis in 21 (30%), 29 (41%) and 20 (29%) subjects, respectively. The sagittal alignment of the lunate demonstrated significant correlation with measurements of the midcarpal and radiocarpal axes with no correlation with its various shape patterns. It is concluded that the relationship between the midcarpal axis and the radiocarpal axis in terms of dorso-volar displacement has a key role in determining the sagittal alignment of the lunate, while the shape of the lunate may be only a secondary consideration.

Carpal bone size and scaling in men versus in women

PURPOSE

The purpose of this study was to quantify carpal bone size, to determine whether gender influences carpal size, and to determine whether small and large carpal bones differ in size only by simple isometric scaling.

METHODS

Cortical surfaces of all carpal bones in both wrists of 14 women and 14 men (ages 22-34 y) were reconstructed from computed tomography (CT) volume images. Carpal volume and bounding-box dimensions in 3 orthogonal directions were calculated and compared across genders. An average set of carpal bones were then scaled mathematically by a single factor in all directions (scaled isometrically) and compared across carpal bones of all sizes.

RESULTS

Although female carpal bones were significantly smaller than male carpal bones, individual carpal volume as a percentage of the volume of the entire carpus did not differ with gender. The 3 orthogonal bounding-box dimensions of the carpal bones scaled nearly isometrically from the smallest to the largest bones.

CONCLUSIONS

Across the wide range of wrist sizes studied the individual carpal volumes were a consistent percentage of carpus volume and this percentage did not differ with gender. Despite their complex shape the bounding dimensions of the carpal bones increased isometrically with increasing volume. The extensive database of dimensions provided in this study should be useful in the design and insertion of fixation systems and implants.

Lunate morphology

Fifty healthy volunteers were subjected to the CT examination of the wrist joint to provide normal database of the shape and size of the lunate. The various parameters of the lunate were measured taking help of the reformatted images in sagittal, coronal and axial planes. The mean maximum antero-dorsal diameter of the lunate measured on axial section was 16.96mm (SD 1.60) with the range of 13-19mm while the mean medio-lateral diameter of the lunate was 12.80mm (SD 1.37) with the range of 10-15mm. The mean axes of the scaphoid and the triquetral articular surfaces of the lunate were 11.83 degrees (SD 9.33) and 1.54 degrees (SD 9.70), respectively, while the mean axial index was 2.04 (SD 1.33). Lunate is reported to have shapes of three different types on plain radiographs. The CT measurements of most lunates failed to classify them into the described three shapes since many lunates showed dissimilar typing on the various chosen sagittal sections of the same lunate. The classical wedged lunate with its apex towards the dorsum has been described to have a tendency to extend under the capitate compressive force. However, in a study on plain radiography no correlation was reported between the radio lunate angle and the shape of the lunate measured in the direction of the lunate's axis. Our study confirmed the same on plain radiographs and on the CT also. We measured lunate's shape in the direction of the capitate's axis too, which demonstrated significant correlation with the RLA (p<0.001).

Change of carpal alignment under anaesthesia: role of physiological axial loading on carpus

OBJECTIVE

to study the effect of physiological axial loading on the carpal alignment.

DESIGN

Patients with distal radial fracture undergoing general anaesthesia for management of their injury were employed for the present study. The contralateral normal wrist was assessed for any change in carpal alignment following the elimination of the physiological axial load under anaesthesia with complete muscle relaxation.

BACKGROUND

Axial load across the wrist is known to affect carpal alignment. Axial compression is known to dorsiflex the lunate due to its dorsally thinned wedge shape. However, cadaveric experiments have also shown the axial compression to flex the whole proximal carpal row.

METHODS

Lateral radiographs of the uninjured wrist joint were performed in twenty patients with distal radial fracture. Radiographs were taken before and after giving general anaesthesia along with muscle relaxants and were repeated after applying the traction in line with the long axis of the radius.

RESULTS

Anaesthesia caused scaphoid and lunate to dorsiflex with no change in scapholunate angle while, traction caused scaphoid to rotate further dorsally.

CONCLUSIONS

Physiological axial loading due to the normal tone of the forearm muscles tends to flex the scaphoid and the lunate. This is contrary to the lunate's classical description of its tendency to dorsiflex under axial loading. Our results also failed to support the view that scapholunate interosseous ligament is in continuous tension due to the tendency of the scaphoid and lunate to move in the opposite directions.

RELEVANCE

A dorsiflexed lunate need not be considered the hallmark for diagnosing the scapholunate dissociation.

Lunate Morphology of Kienböck's Disease on X-Ray Study

To investigate whether the lunate morphology proposed by Watson predisposes to Kienböck's disease, we investigated 23 patients with Kienböck's disease and 24 normal controls. Lateral X-ray films showed 17 of type D (dorsally lower height) lunate; 2 of type P (palmary lower height); and 1 of type N (equal palmar height to dorsal). In the control group, there were 21 of type D; 2 of type P; and 1 of type N, among 24 patients. The mean wedge ratio was 1.17 (1.08-1.35) in type D of the Kienböck group, and 1.11 (1.03-1.41) in type D of the control group. There were no statistically significant differences between the two groups, and palmar and dorsal lunate morphology is not believed to predispose to Kienböck's disease.