Several studies on BMD or strength in trabecular or cortical bone of the metaphysis of the distal radius have been published [11
]. However, to our knowledge no reports have focused on the subchondral plate of the distal radius. We examined the topographic pattern of the subchondral plate of the distal radius and the correlation between various clinical factors and subchondral BMD of the distal radius. We assumed that the subchondral BMD of the distal radius can be affected by (1) systemic or constitutional factors, which include age, height, weight, BMI, and T-score measured by DXA at the axial skeleton; and (2) by local factors that are long-term joint reaction forces represented indirectly by radiographic evidence of osteoarthritis and socio-occupational class.
Our study has several limitations. First, we used CT osteoabsorptiometry, which resulted in densitometric values in HU. Because the HU is the attenuation coefficient that is calibrated with reference to water and not an absolute value, the results produced in HU may present different values depending on CT scanners or examination conditions, leading to a lack of reproducibility in measurement. However, all subjects were evaluated using an identical CT scanner under the same conditions. Therefore, the topographic and correlational analysis in this study would not have substantial problems. Second, the authors used radiographic evidence of osteoarthritis and socio-occupational class as indirect indicators of the accumulated intensity of hand labor and long-term joint reaction force exerted on the distal radius. Although the criteria of osteoarthritis and hard manual worker were based on previous studies [6
], this type of dichotomous way of division is somewhat arbitrary. Third, the estimated densitometric measurement is the maximum value among a series of voxels that are located on the trajectory line because we used the MIP technique. The MIP appears more reasonable, but such a technique is currently unavailable. Nevertheless, selecting the maximum value using MIP will not allow substantial error because the subchondral plate of the distal radius in the elderly is normally very thin.
In the quantification, the mean subchondral BMD of the subjects was 518 HU (range, 360–720 HU). In a previous study, bone quality was categorized into five classes based on the Hounsfield unit obtained from a CT scan [26
]. In this classification, the BMD range of 360 to 720 HU, as in our study, corresponds to the D3 class, which consists of the porous cortical bone and the fine trabecular bone. Considering this, the quantified results of our study showed that the subchondral BMD of the distal radius in postmenopausal women usually corresponds to the interim value between the BMDs of the cortical and trabecular bones, and is relatively widely distributed through the range (eg, 360–720 HU), depending on the subjects.
In the topographic analysis, the gross pattern of the densitometric distribution was assessed and a quantified comparative analysis was performed by dividing the subchondral plate into four subareas (radial half of the scaphoid fossa, ulnar half of the scaphoid fossa, interfossa ridge, and lunate fossa). The subarea division like this was based on the load-bearing situation of each location, ie, the ulnar half of the scaphoid fossa and lunate fossa bear the transarticular (radioscaphoid and radiolunate) load directly but the interfossa ridge does not bear any direct load and the radial half of the scaphoid fossa bears a range of loads depending on the posture of the wrist. Statistically, the areas of the ulnar half of the scaphoid fossa and lunate fossa have a significantly higher BMD than the interfossa ridge, as shown in the gross assessment of each densitometric image (Fig. ). This bicentric pattern of the subchondral BMD can be explained by the hypothesis that the transarticular load of the wrist is transmitted mainly by the radioscaphoid and radiolunate joints and the subchondral plate of the distal radius adapted to this long-standing mechanical strain. This type of densitometric distribution corresponds well with studies by Carlson and Patel [4
]. However, the subchondral BMD of the ulnar half of the scaphoid fossa is significantly higher than the radial half of the scaphoid fossa and lunate fossa, and there was no statistical difference in subchondral BMD between the radial half of the scaphoid fossa and the lunate fossa. This suggests the contact area between the proximal pole of the scaphoid and the ulnar half of the scaphoid fossa is the primary load-transmitting location, and the radiolunate joint and radial half of the radioscaphoid joint are the secondary load-transmitting locations. In addition, this topographic distribution of BMD has several clinical implications. First, when intraarticular extension of the distal radius fracture or lunate die-punch fracture occurs, the interfossa ridge, with a relatively low BMD, is normally involved in fractures. Second, when performing volar plating to a distal radius fracture, locking screws produced better support on a relatively dense subchondral portion of the lunate fossa and the ulnar half of the scaphoid fossa rather than that of the interfossa ridge.
For correlational analysis, several systemic and local factors were assumed to be the clinical factors with a potential correlation with subchondral BMD. Among the systemic factors, only the systemic BMD measured in the T-score by DXA at the lumbar spine and femoral neck was related to the subchondral BMD. In general, several anthropometric variables including age, height, weight, and BMI are believed to be related to systemic osteoporosis in postmenopausal women despite some debate [27
]. However, our study showed such anthropometric variables do not correlate with the subchondral BMD of the distal radius.
The local factors represented by the radiographic evidence of osteoarthritis and socio-occupational class also were found not to correlate with the subchondral BMD of the distal radius. A mechanical load applied to a joint includes the weightbearing force and joint reaction force [29
]. Because humans are bipedal animals, the weightbearing force applied to the wrist can be negligible and there remains only a joint reaction force that is made from the muscle-tendon structure. We assessed the accumulated intensity of hand labor experienced in the past using the socio-occupational classification and radiographic evidence of osteoarthritis to objectively determine the long-term joint reaction force exerted on the distal radius. We used the scheme described by Erikson et al. [12
] as a socio-occupational classification and the Kellgren-Lawrence scale as an osteoarthritis grading system to objectify and minimize the arbitrariness. The scheme by Erikson et al. [12
] has been used to evaluate the socioeconomic factors in a range of disease situations [5
], and the Kellgren-Lawrence scale has been used to evaluate hand osteoarthritis and its impact on the functional status in the study by Zhang et al. [39
]. As a result, the intensity of accumulated hand labor showed no significant relationship with the subchondral BMD of the distal radius.
These negative correlations can be explained by Frost’s mechanostat hypothesis [15
]. There is a range of strain that maintains constant bone turnover. The lowest value of that range is called the minimum effective strain (MES) for the remodeling threshold. Bone loss progresses when the strain applied to the bone is less than the MES, whereas bone gain progresses when the strain is beyond that range. Because the wrist is not a weightbearing joint in humans, gravitational factors such as weight or BMI have little effect on the wrist. In addition, the joint reaction force caused by accumulated intense hand labor might not reach the level of the remodeling threshold. Only the systemic BMD measured in the axial skeleton was found to correlate with the subchondral BMD of the distal radius, possibly as a constitutional factor.
We believe our results provide anatomic and clinical insights regarding the microstructure and mechanical properties of the subchondral plate in the distal radius, which have been relatively undisclosed. Also, the topographic findings in this study may be helpful when performing surgery and for implant development.