PURPOSE:
To evaluate the influence of stem offset and neck shaft angles on the range of motion before component impingement (ROMCI) and bony impingement (ROMBI), and the types of impingement in total hip arthroplasty (THA).

METHODS:
Using the computed tomography data of 101 patients who underwent THA, three-dimensional dynamic motion analysis was performed using a modular implant (Kinectiv(®) stem) that enabled adjustment of offset and leg length independently. We defined offset as horizontal offset (HO) and leg length as vertical offset (VO), and measured the ROMCI and ROMBI in flexion (Flex), internal rotation (Int-R) and external rotation (Ext-R) with the configuration of each horizontal/vertical offset.

RESULTS:
We found that HO lengthening increased the ROMCI and ROMBI in Flex and Int-R by delaying bony impingement, although excessive lengthening had minimal effect. On the contrary, VO lengthening decreased the ROMCI and ROMBI in Flex and ROMCI in Int-R. As for Ext-R, VO lengthening had positive effects on the ROMCI and ROMBI, whereas lengthening of HO had negative effects on the ROMCI and ROMBI.

CONCLUSIONS:
We demonstrated that the appropriate long offset with a low shaft angle increased the ROM in Flex and Int-R, and a high neck shaft angle increased the ROM in Ext-R. We should use implants properly in accordance with the types of impingement for avoiding dislocations in THA.





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