Lunate Fracture Morphology in Kienbock's Disease—A Computed Tomography Study

Abstract

Background  The pathogenesis of Kienbock's disease is poorly understood. The coronal fracture line is acknowledged as a poor prognostic marker in the disease. Other fracture types in the Kienbock's wrist have subsequently been identified.

Questions/Purposes  The aim is to study the fracture morphology of the lunate in a cohort of patients using computed tomography.

Methods  Patient images were acquired using a set protocol with four-dimensional computed tomography scanning. Images were reviewed by two orthopaedic surgeons and a consultant radiologist. Static and dynamic images were assessed and a fracture map created. The relationship of fracture type to other parameters was then analyzed.

Results  Twenty-three patients were included in the study, including 11 males and 12 females, with a mean age of 43 years. Total frequency of fracture lines in the cohort was coronal – 26, proximal subchondral bone plate type – 24, avulsion – 19, sagittal – 16, and distal subchondral bone plate type – 11. There were statistically significantly more proximal than distal subchondral bone plate fractures ( p  = 0.03), and more coronal fractures than distal subchondral bone plate fractures ( p  = 0.01). There were statistically significantly more radiolunate ligament-avulsion types ( p <0.001) than other types. The sagittal fracture line through the lunate approximated closely to the ulnar edge of the capitate and the ulnar edge of the radius.

Conclusion  Study on the fracture morphology in the Kienbock's wrist has improved our understanding of the disease pathogenesis. Fracture lines may correspond to loading points, intrinsic and extrinsic ligament avulsions. These fracture types may play a role in disease progression and are important to identify when considering lunate salvage surgery.

The pathogenesis of the Kienbock's wrist is poorly understood. Lichtman described radiographic stages of the disease, including fragmentation and collapse. ¹ He also modified his classification, adding the “coronal” fracture – as a poor prognostic marker. ² Since this description, much emphasis has been placed on this fracture pattern in guiding management. ^{3 4 5}

More recently, other fracture types have become recognized in the Kienbock's lunate, including the sagittal fracture and ligament-attachment fractures. ^{6 7} Comprehensive understanding of the fracture morphology in the Kienbock's wrist is likely to improve our understanding on the disease pathogenesis, help refine our treatment, and understand our failures.

The aim of this study was to study the fracture morphology of the lunate in a cohort of patients using computed tomography.

Methods

Approval was obtained from the Human Research and Ethics Committee. Patients were included if previously diagnosed with Kienbock's disease, requiring further intervention and having failed non-operative treatment.

Images were acquired using a 320-slice multidetector CT scanner (Aquilion One, Toshiba Medical Systems, Inc., Tochigi Ken, Japan). Settings for the scanner, patient set-up, and X-ray protection followed our previously published protocol. ⁸ The investigator was positioned behind the scanner, and passively guided the patient through the movement series, first on the asymptomatic wrist, used as a control, followed by the symptomatic wrist. The wrist was taken slowly through a standard plane of motion with movements including: flexion-extension, radial-ulnar deviation, clenched fist, dart thrower's and anti-dart thrower's motion, and pronation-supination.

Multiplanar reformations were produced using the bone algorithm in coronal, sagittal, and axial planes, and soft tissue reconstructions were used to produce static and dynamic images, including two-dimensional (2D), three-dimensional (3D), and four-dimensional (4D) images (i.e., 3D “movies”). Slice thickness was set at 2 mm. Static and dynamic images were reviewed by two orthopaedic hand surgeons and a consultant musculoskeletal radiologist.

Static and dynamic findings were recorded on both plain radiographs and CT scan. ⁹ Patient demographics and the Lichtman osseous grade of each wrist was determined. ^{1 2}

Data collected for each patient included:

• Lunate fracture : number, morphology.

• Static findings: wrist and lunate morphology, carpal alignment, instability, and Lichtman grade.

• Dynamic findings: instability and impingement types.

Lunate Fracture Assessment

Fracture lines were identified on review of the 2D CT images ( Fig. 1 ). A view of the lunate in the coronal, sagittal, and axial planes were used from Complete Anatomy software (3D4 Medical, Elsevier, Netherlands) and acted as templates for fracture transcription. A scale was drawn for the “x,” “y,” and “z” axes of each aspect of the bone. The image of the lunate from the CT was transposed over this axis and expanded to fit the axes evenly. This lunate image was then made semi-transparent. This allowed fracture lines to be transcribed onto each lunate template using Keynote software (Apple, CA). The position of the fracture lines to associated ligamentous insertion points was determined using the work of Berger. ¹⁰

Fig. 1.

Identification of fractures on two-dimensional computed tomography reformatted images. ( A ) Sagittal image: white arrow showing proximal SCBP fracture, green arrow showing coronal fracture. ( B ) Axial image: green arrow showing volar lunotriquetral attachment fracture, white arrow showing radiolunate ligament attachment fracture. ( C ) Coronal image: white arrows showing sagittal fracture line closely associated with the ulnar edge of the radius and capitate. ( D ) Coronal image: white arrow showing proximal SCBP fracture. SCBP, subchondral bone plate.

Once the mapping process was complete for each case, the fractures were superimposed to create a “fracture map.” When these overlapped, the color became more opaque, creating a heat map. This allowed for better visual representation of the most commonly fractured areas.

A fracture line was defined as “coronal” or “sagittal” depending on its orientation if it propagated from the distal to proximal articular surfaces of the lunate. The location of the sagittal fracture line in relation to the position of capitate and radius was calculated with the wrist in neutral radio-ulnar deviation ( Fig. 2 ).

Fig. 2.

Mapping of the sagittal fracture line in the coronal plane – volar aspect. A template of the lunate was created to allow transcription of the fracture line ( thick white line ). The location of the lunate in relation to the ulnar edge of the capitate ( blue line ) and ulnar edge of the radius ( green line ) was transcribed through the axes. The difference between these lines and the proximal and distal sagittal fracture line at the lunate was then calculated; distally: capitate-lunate (C-L), proximally: lunate-radius (R-L). Value in percentiles.

Fracture lines that involved the subchondral bone plates were defined as either proximal or distal “subchondral bone plate fractures” (SCBP).

If fracture fragments were associated with an interosseous ligament attachment these were defined as “ligament attachment fractures.” These fractures were further defined as those involving the radiolunate ligaments (RLL) dorsal or volar lunotriquetral (LTL) or scapholunate ligament-type.

Static Assessment

Ulnar variance was measured as the distance between a line perpendicular to the longitudinal axis through the distal ulnar aspect of the radius and the distal cortical rim of the ulna. ¹¹ Lunate morphology was classified as either Viegas type 1 (with no medial facet), or type 2 (with a medial facet for articulation with the hamate). ¹² Scapholunate diastasis was defined as a perpendicular measurement between the scaphoid and lunate >3 mm. ¹³ A radioscaphoid angle >60 degrees on a lateral image/sagittal CT was determined as a sign of carpal collapse. ¹⁴ Degenerative changes were assessed on both plain radiography and CT scan. Ulnar translocation was measured and defined using the Bowman ratio. ¹⁵ Ulnar translocation was further defined as by Taleisnik type. ¹⁶ Carpal height ratio was defined using the Nattrass method, ¹⁷ with a normal range of 1.57 ± 0.05.

Dynamic Assessment

On the dynamic 4D-CT scan cine-images were identified with several instability and impingement patterns as previously described. ⁶

The “nutcracker effect ” was defined as dynamic proximal capitate migration into a fractured lunate causing separation of volar and dorsal fragment. ^{6 7} This was determined on dynamic 2D sagittal images with flexion-extension and 2D coronal images with radial- and ulnar-deviation.

• Volar radiolunate impingement was defined on dynamic 2D sagittal images as mechanical impingement of the volar horn fracture fragment of the lunate with wrist flexion-extension.

• Ulnar styloid-triquetral impingement was defined as mechanical impingement of the triquetrum against the ulnar styloid on dynamic 2D coronal views of the wrist in ulnar-radial deviation. ¹⁸

• We defined dynamic proximal row instability as dynamic scapholunate diastasis or LTL instability on dynamic 2D coronal imaging. ⁶

• Dynamic internal lunate instability was defined as dynamic separation of lunate fragments >1 mm. This phenomenon was identified on dynamic 2D axial, sagittal, and coronal images during dynamic 3D coronal sequences. ⁶

Statistics

Associations between variables were tested statistically using non-parametric Mann-Whitney U-tests, Friedman's and Chi-square or Fisher's exact tests as appropriate to the variable types. A two-tailed p -value <0.05 was taken to indicate statistical significance.

A post-hoc power analysis of fracture type frequency showed that the study had an 86% power to detect a 40% difference per fracture subtype, with an α rate of 0.05.

The senior authors repeated their measurements and intraclass correlation coefficient was 0.964 (95% CI 0.916–0.985) suggesting excellent agreement for intraobserver error.

Results

In total, 23 patients were included in the study. There were 11 males, 12 females. Mean age was 43 years (16–75). Lichtman grades of the patients were; II – 5, IIIA – 6, IIIB – 2, IIIC – 5, IV – 5. Twenty-two of 23 patients had a Viegas-type 1 lunate. Eighteen of 23 patients had ulnar negative variance.

Lunate Fracture Assessment

Fracture line morphology in the lunate was compared. Heat maps of fracture distribution in all cases from different aspects are seen in Fig. 3 . Total frequency of fracture lines in the cohort was coronal – 26, proximal subchondral bone plate type – 24, avulsion – 19, sagittal – 16, distal subchondral bone plate type – 11. There were statistically significantly more proximal than distal subchondral bone plate fractures ( p  = 0.03), and more coronal fractures than distal subchondral bone plate fractures ( p  = 0.01). There were no statistically significant differences between frequency of other fracture type comparisons ( Fig. 4 ).

Fig. 3.

Heat map created with the multiple fractures all on the same template. The fracture distribution in the cohort on sagittal, coronal, and axial planes. (D, dorsal; V, volar; R, radial; U, ulnar)

Fig. 4.

Box-and-whisker plots showing comparison of fracture type in the cohort. ( Black cross corresponds to mean value. Points above or below the whiskers upper and lower bounds are outliers).

Subgroup analysis of ligament-attachment fractures was performed ( Fig. 5 ). There were statistically significantly more RLL-avulsion types ( p <0.001) than other types.

Fig. 5.

Incidence of ligament-attachment fractures within the cohort.

The Sagittal Fracture

The ulnar edge of the capitate was positioned at a mean 75 ^th percentile on the distal-ulnar aspect of the lunate (95% CI 69–81). The distal sagittal fracture line was positioned at a mean 74 ^th percentile on the distal-ulnar aspect of the lunate (95% CI 69–80). The mean difference was one percentile (95% CI −3–4).

The ulnar edge of the radius was positioned at a mean 84 ^th percentile on the proximal-ulnar aspect of the lunate (95% CI 81–86). The proximal sagittal fracture line was positioned at a mean of 82nd percentile on the proximal-ulnar aspect of the lunate (95% CI 79–85). The mean difference was two percentiles (95% CI 1–4) ( Fig. 6 ).

Fig. 6.

Results of mapping of the sagittal fracture line in the coronal plane – volar aspect. Value in percentiles.

Lunate Fracture Association with Static and Dynamic Pathology

Static and dynamic findings were analyzed for each fracture type ( Table 1 ). Increasing number of fragments were associated with increased radioscaphoid angle ( p  = 0.04), presence of ulnar translocation ( p  = 0.02), and a higher ulnar translocation index ( p  = 0.001). A higher Lichtman grade was associated with the presence of proximal SCBP fractures ( p  = 0.02). Distal SCBP fractures were associated with an increased ulnar carpal translocation index ( p  = 0.005). Proximal SCBP fractures were associated with volar radiolunate impingement ( p  = 0.048).

Table 1. Association of fracture type with cohort demographics, static and dynamic findings ( p -values shown) .

	Fracture type
Demographics	Coronal	Sagittal	Prox SCBP	Distal SCBP	Avulsion	Number of fragments
Static findings
Ulnar negative	0.34	0.17	1	0.12	0.36	0.20
Viegas type 1 Lunate	0.42	0.46	0.26	1	1	0.09
Increased RSA	0.34	1	0.58	0.27	1	0.04
SL Diastasis	1	1	0.62	1	1	0.32
Degenerate RL joint	1	0.68	0.62	1	0.39	0.93
Degenerate RS joint	0.67	0.36	1	1	0.65	0.81
Degenerate STT joint	0.41	0.22	0.64	0.67	0.68	0.38
Degenerate LC joint	1	1	0.64	1	0.21	0.83
Ulnar translocation present	0.27	0.30	0.54	0.07	0.60	0.02
Ulnar carpal translocation index	0.15	0.21	0.12	0.005	0.12	0.001
Decreased carpal height ratio	0.31	0.28	0.07	0.58	0.08	0.18
Lichtman grade	0.07	0.79	0.02	0.10	0.63	0.23
Dynamic findings
Nutcracker effect	1	0.11	0.34	0.65	0.97	0.61
Volar radiolunate impingement	0.68	0.41	0.048	0.36	0.24	0.25
Ulnotriquetral impingement	0.62	0.63	1	1	0.33	0.49
Dynamic proximal row instability	1	1	0.06	1	1	0.07
Internal lunate instability	0.68	1	1	1	0.21	0.64

Abbreviation: SCBP, subchondral bone plate.

Note: Values given in bold are statistically significant with a p -value <0.05.

Discussion

Much importance has been placed on the coronal fracture in Kienbock's disease. Bain described the fracture on arthroscopic assessment and developed a treatment algorithm for management. ^{19 20} Lichtman modified his original classification based on this – with the addition of 3C in his grading, and recommended excision of the fragmented lunate and proceed to STT fusion or proximal row carpectomy. ² Our recent work has highlighted the complex kinematics of the Kienbock's wrist, particularly the presence of different fracture types – each involving separate important aspects of the lunate, including load-bearing areas (subchondral bone plate), ligament attachment points, and areas exposed to cantilever force (sagittal fracture). Aside from the coronal fracture, this study has shown a similar incidence of many of these other important fracture types in Kienbock's disease.

Proximal subchondral bone plate fractures were seen in almost equal frequency to coronal fractures. These were also associated with more advanced Lichtman grade. Collapse of the subchondral bone proximally leads to loss of congruity between the lunate and lunate fossa. As a result, we found there was increased risk of volar radiolunate impingement in these patients, as the lunate hinges in the proximal subchondral defect when moving from extension to flexion as seen on dynamic imaging. Volar radiolunate impingement was also identified in patients with fractures involving avulsion of the volar horn radiolunate ligament attachments ( Fig. 7 ). Proximal SCBP fractures were identified significantly more frequently than distal SCBP fractures. Micro-CT study has shown that the proximal SCBP is only a single trabecular layer thick and therefore likely more prone to failure with repetitive loading. ²¹ The distal subchondral bone plate is thicker on micro-CT, hence we found fracture of the distal subchondral bone plate to be less common in our cohort.

Fig. 7.

Two mechanisms for volar radiolunate impingement. ( A ) Collapse of the proximal SCBP: Congruity is lost. The lunate hinges on attempted flexion. Note the corresponding geode on the lunate fossa. ( B ) Radiolunate ligament avulsion: the fragment abuts the lunate fossa on flexion. SCBP, subchondral bone plate.

There were 16 sagittal fractures identified in the cohort. The fracture line propagated in very close approximation to the ulnar edge of the capitate and the ulnar edge of the radius. Viegas-type 1 lunates tend to be smaller, trapezoidal and overhang the ulnar edge of the lunate fossa. ⁷ The majority of patients in our cohort had “at-risk” morphology, with Viegas-type 1 lunates and ulnar-negative variance. We have previously described the “nutcracker effect” of the wrist on this “at-risk” lunate. ⁷ The lunate is loaded by the capitate with counter-load from the radius. A cantilever force is produced on the lunate with radial-ulnar deviation. Force propagates between the stressed and unstressed areas of the bone – i.e., at the ulnar edge of the adjacent radius and capitate. We therefore believe that the sagittal fracture in Kienbock's disease is primarily a biomechanical consequence of this repetitive loading pattern.

We found a high incidence of “ligament-attachment fractures.” The pathogenesis of Kienbock's disease advanced collapse is unknown. Later disease stages have been described by Lichtman as progression from IIIA – with some fragmentation or collapse of the lunate but normal carpal alignment, to IIIB –carpal instability with increased radioscaphoid angle. Stage IV involves progression of intercarpal arthritis to involve the radioscaphoid joint. Instability within the Kienbock's wrist is a complex phenomenon. The pathogenesis leading to progression from IIIA to IIB is, however, unknown, and the cause for carpal collapse with instability is complex. Synovitis is shown to be a consistent finding on arthroscopic examination in Kienbock's disease, and ligament attenuation or rupture can occur. ^{19 22} Our study clearly shows that the ligament failure in the Kienbock's wrist commonly includes the osseous insertion – involving both intrinsic interosseous ligaments, as well as extrinsic radiocarpal attachments. The majority of these involved the attachment site of the short- and long-radiolunate ligaments on the volar horn of the lunate. Larger cohort studies or laboratory studies will allow us to identify which of these fragments or which combination is the key to progression of carpal instability and failure. Ulnar translocation appears to be a common finding in late stage disease and was associated with a higher number of fracture fragments. ⁶

We had limited numbers in our study, reflecting the rare nature of the disease and the exclusion of patients successfully managed non-operatively. Despite close study of the thin-slice CT scans, it is likely that some fracture lines may have been difficult to identify in some cases with significant fragmentation. We were unable to identify each ligament on the CT scan, and therefore association of fractures to specified ligament attachments was made based on agreement of the observers.

Study on the fracture morphology in the Kienbock's wrist has improved our understanding on the disease pathogenesis and helps refine our treatment. We have identified the sagittal fracture occurring at an “at-risk” area of an “at-risk” lunate. Ligament-attachment fractures are likely to contribute to the complex pathogenesis of disease progression with carpal instability and collapse. Instability commonly involves ulnar translocation of the carpus in advanced stages of disease. These findings have clinical relevance on management. Biomechanical and vascular risk factors play a role in disease etiology. Primarily, lunate-preserving surgery needs to optimize the biomechanical loading conditions on the lunate to prevent fracture, as fracture in the coronal or sagittal plane in an ischemic or necrotic bone in a compromised mechanical environment is unlikely to heal. Before lunate-preserving surgery is undertaken, advanced CT imaging is recommended to assess for small avulsion fractures in axial, coronal, and sagittal planes, which may preclude carpal instability and collapse. We would advise against a complex vascularized bone graft procedure in cases with ligament-attachment fractures, as the risk of carpal instability and poor functional outcomes is likely to be substantial. We would also recommend the use of 4D CT imaging in select cases to assess fracture pattern and associated dynamic instability.