Purpose

To map ideal patella tunnel placement with the use of preoperative CT scan and compare results after MPFL reconstruction with postoperative CT scans between a) a free-hand, hardware-free patellar fixation technique with two semi-patellar tunnels and b) its modification with the use of an ACL aiming tibia device.

Materials And Methods

30 knees from 15 fresh frozen human cadavers (15 left and 15 right) were randomly allocated to 2 groups. Group A consisted of knees operated with the free-hand, hardware-free technique and Group B consisted of knees operated on with a technique modification with the ACL aiming tibia device.
CT scan was performed on all specimens a) prior to any intervention (with ideal tunnel mapping) and b) after MPFL reconstruction with the two techniques.
Preoperative tunnel mapping (ideal tunnel placement):
Midcoronal section definitions: L1=maximal patellar length, D1=distal tunnel entry point and D2= proximal entry point. Consequently, D1-D2 distance is maximum possible distance between the entry points of the two tunnels at the medial patella margin. L2=distance of proximal pole from proximal tunnel. Working length for this surgical technique is the bone bridge between tunnels and was calculated as L1/2-L2. The correlation coefficient (r) was calculated for L1 and D1-D2.
MidAxial section definitions: A=point of anterior cortex at the lateral patella margin, C=point of cartilage at the lateral patella margin. R1=angle between A-D1-R and R2=angle between A-D2-R. R angle represents safe angle at the entry point during tunnel drilling in MPFL reconstruction (without breaching the anterior cortex or articular cartilage).
Postoperative measurements: evaluation of patella tunnels placement with the two techniques compared with preoperative planning.

Results

In the preoperative measurements from 30 knees, mean L1 was 3.45cm (range 3.05-4.52, SD 0.39). Mean L2 was 0.62cm (range 0.49-0.89, SD 0.12). The mean maximum possible bone bridge between tunnels (L1/2-L2) was 1.1cm (range 0.77-1.58, SD 0.21) and D1-D2 distance was 1.19cm (range 0.95-1.67, SD 0.24). L1/2-L2 was significantly shorter than D1-D2 (p=0.035). The correlation coefficient for L1 and D1-D2 was r=0.78 and for L1 and L1/2-L1 it was r=0.82. Consequently, bone bridge between tunnels in patellas shorter than 3.2cm, was less than 1 cm. R1 was 6.050 (range 4.78-7.44, SD 0.92), R2 was 6.640 (range 4.57-9.03, SD 1.03), and their difference reached statistical significance (p=0.03). Postoperatively, in group A, in 4/15 patellas multiple attempts were made during tunnel drilling in order to avoid anterior cortex or cartilage breaching. In group B, all tunnels were correctly drilled with the first attempt. Bone bridge between tunnels was significantly shorter postoperatively (=0.93, p<0.01). All tunnels in both groups were drilled in a parallel fashion.

Conclusion

The smaller the patella, the smaller the bone bridge between tunnels, making hardware-free patellar fixation technique with two semi-patellar tunnels during MPFL reconstruction challenging for small patellas. Furthermore, R angles create a narrow window to avoid intraoperative breaching, rendering the use of the ACL tibia device an extremely useful instrument.