Balancing the patellofemoral joint (PFJ) in total knee arthroplasty (TKA) involves avoiding over-stuffing. The purpose of this study was to assess how often a strategy of recreating the anterior space of the trochlea (full extension) led to the trochlea depth being recreated in both mid-flexion (30-40°) and deep flexion (80-90°). One hundred and twenty two consecutive patients undergoing robotic-assisted TKA had femoral components placed according to functional alignment principals and were assessed. The femoral component was sized and positioned in order to ensure that the anterior flange was within 2 ​mm of the native anatomy, corresponding to a patella position of full extension (0° flexion). Trochlea depth restoration in 3 positions along the floor of the trochlea groove was compared and measured. The trochlea was defined as balanced if the prosthesis was within 2 ​mm of the native anatomy. Patients were divided into over-stuffed (prosthesis >2 ​mm above the native anatomy) or under-stuffed (prosthesis >2 ​mm beneath the native anatomy). All patients 122/122 (100%) had a balanced trochlea in full extension. In total 54 TKA were over or under-stuffed at either mid-flexion or deep flexion. In mid-flexion, 3/122 (2.5%) trochlea were over-stuffed and 39/122 (32%) trochlea were under-stuffed. In deep flexion, 25/122 (20.5%) of trochlea's were overstuffed and 30/122 (24.6%) were under-stuffed. In mid-flexion, balanced trochlea components were more externally rotated relative to the posterior condylar axis compared to unbalanced components (2.35° external rotation vs 1.21°, p=0.004). There were no other significant differences observed between the balanced and unbalanced trochlea groups in mid or deep flexion. Over 40% of TKA over or under-stuff the trochlea in deeper flexion despite the anterior flange being positioned within 2 ​mm of the native anatomy in full extension. The rate of over or under-stuffing in mid and deep flexion was similar (>40%); however, in mid-flexion, under-stuffing of the native trochlea was more common. The concept of PFJ over or under-stuffing in TKA needs to be redefined to consider the full arc of flexion of the trochlea groove, and the biomechanical and clinical consequences of under-stuffing the trochlea investigated further. Level IV.

Functional dissatisfaction following total knee replacement (TKR) is recorded as high as 20%. The majority of these patients report anterior knee pain (AKP) as the main source of dissatisfaction. Elevated patellofemoral compression forces and soft tissue extensor hood strain have been implicated in the generation of significant AKP. A novel method of assessing and measuring patellofemoral compression forces dynamically in the native and resurfaced patella for TKR in four different quadrants of the patella is described. Results are reported from an in vitro model and cadaveric studies in the native and resurfaced knee. Patellofemoral compression forces are shown to be characteristic and consistent over repeated assessments in the native knee. Placement of a TKR significantly alters this pattern. Furthermore, over-stuffing or under-stuffing the resurfaced patella also significantly alters the nature and magnitude of patellofemoral compression forces. These studies may lead to an improved understanding of the nature of AKP following TKR, and using this assessment tool presents an opportunity to more effectively balance the third space, reproduce the native patellofemoral forces, and subsequently reduce AKP following TKR.