Purpose
Proximal tibia deformity is the main cause of lower extremity varus deformity and it can aggravate knee osteoarthritis (KOA). There are reports that osteoporosis affects the deformation of the proximal tibia, but no report on why the deformation of the proximal is more common than that of distal femur. The purpose of this study is to reveal that the deformation of the tibia is more common than the femur due to the difference in bone mineral density (BMD)
Materials And Methods
We retrospectively analyzed patients who underwent TKA for end-stage KOA. Only patients with varus alignment were included, and patients who underwent surgery that could affect alignment were excluded. Preoperative Dual Energy X-ray absorptiometry (DEXA) and knee computed tomography (CT) were performed in all patients. In knee CT axial cut, housefield unit (HU) of distal femur and proximal tibia were measured form the 2cm from the knee joint and measurements were divided into medial and lateral side. Medial proximal tibial angle (MPTA) was measured and divided into a group with more 85° (Group 1) and a group with less than 85° (Group 2), and Each DEXA T-score and HU were compared. In addition, to determine whether MPTA collapse due to the difference in BMD between distal femur and proximal tibia, the difference in HU between distal femur and proximal tibia was compared in both groups.
Results
A total of 193 patients were included, 112 in Group 1 and 91 in Group 2. The HU of the femur was higher than that of the tibia (femur medial: 144.44±50.74, femur lateral: 171.48 ±61.73, tibia medial: 121.858 ±43.13, tibia lateral: 73.61±44.76, P<0.01). In order to reduce the error cause by MPTA collapse, only group 1 was analyzed again, and the results were the same (femur medial: 154.27±56.92, femur lateral: 179.18 ±66.46, tibia medial: 120.71±48.98, tibia lateral: 80.81±47.66.76, P<0.01). In both groups, DEXA showed no difference in L-spine T-score, but femur T-score was lower in group 2 (femur neck: Group 1 0.90 ± 1.19, Group 2 -0.13 ± 1.10, P=0.016, femur total: Group 1 -0.53 ± 1.31, Group2 -0.89 ± 1.15, P=0.049). Femur lateral, femur total and tibia lateral HU was also low in group 2 (femur lateral: Group1 180.63±65.46, Group 2 158.82±54.05, P=0.015, femur total: Group1 165.21±59.04, Group2 147.70±45.36, P=0.027, tibia lateral: Group 1 80.16±47.98, Group 2 64.57±38.34, P=0.017). There was no difference in HU difference and MPTA collapse.
Conclusion
The reason that the medial tibia collapses more than the medial femur in varus alignment end-stage knee OA is that the BMD of the proximal tibia is lower than the BMD of the distal femur. In the MPTA collapse group, the T-score of DEXA femur and HU of CT were low. Also, the collapse of MPTA was affected by the absolute value of BMD rather than the BMD difference between distal femur and proximal tibia.