The third gap - The forgotten space in total knee arthroplasty.

Aims: Total Knee Arthroplasty (TKA) aims to leave the proximal flange of the femoral component flush with the femoral cortex. Manually, the requisite plane is found using the anterior femoral cortex or the intramedullary canal, whereas navigation uses hip and knee centre. Presently, no system prioritises restoration of the third space or native trochlear groove height (TGH) and there is a deficiency of published data on the variation of TGH with respect to the anterior cortex. This study aims to address this deficit. Hypothetically, restoration of the third space occurs when trochlear component depth equals TGH.

Materials And Methods: Relative to the posterior femoral axis the height of the anterior femoral cortex is higher laterally than centrally. For simplicity, this study reports MRI measurements of TGH relative to the centre in 110 normal subjects.

Results: TGH varied from the anterior femoral cortex by a mean of 2.32 mm (standard deviation, SD 1.77 mm, range -1.50 mm to 6.80 mm). If a femoral component trochlear depth of 2.2 mm is assumed, then 24.5% would be either over- or understuffed by more than 2 mm.

Conclusion: There is significant variation in TGH relative to the anterior femoral cortex. Assuming a femoral component trochlear depth of 2.2 mm, approximately one quarter of patients (24.5%) will be over- or understuffed by more than 2 mm. Variation in femoral component flexion and extension combined with whether or not it is left proud or notched will add further variation. Failure to restore the third space is likely to contribute to unsatisfactory results following TKA. Future surgical workflows should address this.