The clinical significance of different lunate types has recently been highlighted. Lunate morphology can be classified by the absence (Type I) or presence (Type II) of a medial facet that contacts the proximal pole of the hamate [3]. Several studies have reported that this additional articulation can alter load transmission across the radiocarpal joint and exert significant influence on carpal kinematics [4, 6, 7] A protective effect against carpal instability associated with scaphoid nonunions and scapholunate dissociation in Type II lunate has been reported [10]. Similarly, Rhee et al. [5] reported that Type II lunate can inhibit abnormal scaphoid flexion deformity in Kienböck disease and may slow the progression of lunate fracture and fragmentation. Lunate classification according to the medial facet has used simple radiographs, but few studies of the reliability of this method are available.
Plain radiographic prediction of lunate morphology has some limitations [9]. Several studies have suggested radiographic characteristics of adjacent carpal bone shapes and alignment to determine the lunate type; however, their interpretation still depends on eidetic impressions of the presence of a medial facet on the lunate. Further, a small difference in the CTD might be the result of inherent bone size differences or measurement errors. The current study assessed intra-observer reliability and inter-observer agreement of two commonly used classification systems for radiographic lunate types and their compatibility with MRA images. Both systems had moderate intra- and inter-observer reliabilities; nevertheless, the CTD analysis showed more consistent agreement than did the PA radiographic analysis. The estimated Kappa value for inter-observer reliability was 0.554 with 95% confidence interval of 0.487 to 0.620. This suggests that the true value of Kappa is higher than 0.487 with 95% confidence. The overall compatibility with MRA for plain radiographic prediction was ~ 79.3%. When the lunates were interpreted to have a medial facet (Type II) on radiographic analyses, they showed good compatibility with the MRA findings. However, when the lunates were interpreted as Type I, there were more cases of mismatch, in which undetected medial facets were observed on MRA.
A possible explanation for the discrepancy in the lunate type on plain radiographs, especially among observers, is that variable size and geographic differences in the medial facet of the lunate may be confusing to some observers. Specifically, the observers had difficulty in interpreting a smaller medial facet, which sometimes overlapped with a lateral facet for capitate articulation and the ulnar margin of the lunate on the PA view. Although the anatomical diversity of the medial facet of the lunate is not fully understood, we observed several discrepancies among the observers when the angle of the medial facet with the lateral facet was less acute. To add to this difficulty, the medial facet is often larger dorsally and may be completely absent on the most volar side. A cadaveric study showed 77% occupation of the lunate medial facet. Pfirmann et al. [17] reported that the size of the medial facet is not uniform throughout consecutive coronal images on MRA. Some of the medial facets did not occupy the entire anteroposterior width of the lunate and were absent in the most volar coronal images. When the lunate occupied a lesser anteroposterior width, the radiographic articular line may not have clearly reflected the presence of a medial facet. Thus, although the lunate types looked clear pictographically, the types were sometimes interpreted subjectively, leading to variability in the inter- and intra-observer reliabilities.
The incidence of Type II lunates seems to vary in different populations [5]. The rate has been reported to range from 27 to 73% worldwide. In the largest study of an Asian population, Tatebe et al. [19] reported a 45% incidence rate in 637 cases of midcarpal arthroscopy. Arai et al. [20] reported a 57.5% incidence of Type II lunates in 127 cadaveric wrists. The incidence of Type II lunates in our study was 42.7%, which is similar to that of previous reports. Most incompatible cases were Type I lunates determined via PA radiographic analysis; in contrast, Type II lunates showed an almost perfect match between the PA radiographic analysis and MRA findings. This study showed a 92.7% compatibility rate of Type II lunates with the MRA findings; Type I lunates showed a lower compatibility rate of 74.3%. These findings suggest that clinicians should be alert to undetected medial facets of the lunate in patients with Type I lunate determined by PA radiographic analysis. In the CTD analysis, Types I and II both showed good compatibility with the MRA findings, and the evaluations by the same observer were more consistent. Both Type I lunate (CTD ≤ 2 mm) and Type II lunate (CTD ≥ 4 mm) showed a strong match with the MRA findings. However, we could not determine the superiority of the CTD analysis to PA analysis. More than half of the patients were classified into the intermediate group in the CTD analysis, and this group showed a similar distribution of Type I and Type II lunates on MRA. The clinical effects of an intermediate lunate type are unknown; thus, further studies are warranted to examine the effects on wrist pathology or kinematic characteristics of the intermediate type.
Several limitations of this study need to be acknowledged. Although we used static images, which did not allow simulation of carpal motions, the actual articulation of detectable facets was not verified. Instead, the articulating facets were predicted from MRA, which provided serial slices of the thinly sectioned articular surface and cartilage shape. Although the original classification of lunate type, derived from a cadaveric dissection, included a medial lunate facet width of less than 1 mm [3], we used a stricter definition for a medial hamate facet in accordance with a previous MRA study [17]. There is still controversy as to whether lunates with a very small facet have clinical significance. Although it is possible that we underestimated the number of Type II lunates, it should be noted that the proportion of Type II lunates evaluated using MRA in our cohort was similar to that of previous studies of the same ethnicity [19, 20].