A computed tomography cadaveric study of the radiological anatomy of the patella: the size of the patella correlates with bone bridge between tunnels and R angles are introduced for safe tunnel drilling during MPFL reconstruction

Abstract

Purpose: To measure the safe range of angles during tunnel drilling and map ideal patella tunnel placement with the use of preoperative computed tomography (CT) scan and compare results after medial patellofemoral ligament (MPFL) reconstruction using a hardware-free patellar fixation technique with two semi-patellar tunnels between a) a free-hand technique, and b) its modification with the use of an anterior cruciate ligament (ACL) tibia aiming device.

Methods: CT scan was performed on 30 fresh-frozen cadaveric knees a) prior to any intervention and b) after MPFL reconstruction. For MPFL reconstruction, specimens were randomly allocated to 1) Group A, which consisted of knees operated with free-hand, hardware-free patellar fixation technique with two semi-patellar tunnels and 2) Group B, which consisted of knees operated on with a technique modification with the ACL tibia device.

Patellar measurements: L1 was the maximal patellar length. L2 was the minimum possible distance of placement for the upper tunnel from the proximal pole of the patella. The maximum bone bridge between tunnels was calculated as half of L1 minus the L2 distance (L1/2-L2). We also measured R1 and R2 angles at the proximal and distal tunnel that represent safe angles at the entry point during tunnel drilling (without breaching the anterior cortex or articular cartilage).

Results: Preoperatively, mean L1 was 3.45 cm (range 3.05-4.52). Mean L2 was 0.62 cm (range 0.49-0.89). The mean maximum possible bone bridge between tunnels (L1/2-L2) was 1.1 cm (range 0.77-1.58). R1 was 6.05⁰ (range 4.78-7.44), R2 was 6.64⁰ (range 4.57-9.03), and their difference reached statistical significance (p = 0.03). Postoperatively, in group A, in 4 out of 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 cm, p < 0.01).

Conclusion: Small-size patellae correlate with short maximum bone bridge between tunnels, which makes anatomic, double-bundle, hardware-free patella fixation, with two semi-patellar tunnels MPFL reconstruction challenging. Furthermore, R angles create a narrow window to avoid intraoperative breaching, rendering the use of the ACL tibia device an extremely useful instrument.

Level of evidence: II.

Keywords: Cadaveric; Computed tomography; Double-bundle MPFL; Patella instability; Patella tunnels; Radiological anatomy.

Fig. 1

Preoperative planning. a Midsagittal section of the patella, blue and purple axis intersect at distal tunnel direction. b Midaxial section of the patella, F is the point of the rim of the anterior cortex at the lateral patella margin and C is the point of the rim of articular surface at the lateral patella margin. R1 is defined as the angle between F-D1-C. c Midcoronal section of the patella, L1 is the maximal patellar length. W is defined as patellar width (perpendicular to and at midpoint of L1). D1 is the distal tunnel entry point (where W touches the medial patellar margin) and D2 is the proximal tunnel entry point. d Midsagittal section of the patella, blue and purple axis intersect at proximal tunnel direction. e R2 is the angle between F-D2-C. f Wf is patella width at the point which a line parallel to W reaches the anterior cortex of proximal patella pole. D2 is the proximal tunnel entry point (where Wf touches medial patella margin). L2 iss the distance of proximal patella pole from Wf

Fig. 2

Preoperative planning (2). The maximum bone stock between tunnels is calculated as half of L1 minus the L2 distance (L1/2-L2). Consequently, D1-D2 distance is the maximum possible distance between the entry points of the two tunnels at the medial patella margin that the surgeon can perceive by palpation in real time surgical conditions

Fig. 3

Graph, correlation between L1 and L1/2-L2. y axis is L1 and x axis is L1/2-L2. Perpendicular red dotted line represents the maximum bone bridge between tunnels equal to 1 cm and horizontal dotted line represents patella length equal to 3.25 cm. The blue line represents the correlation coefficient. Patellas shorter than 3.25 cm correspond to bone bridge shorter than the “desired” 1 cm length

Fig. 4

Sagittal, frontal and axial CT section of patella #7 postoperatively. Tunnel placement in a and b appears adequate. However, in c, multiple attempts (with breaching of the articular surface and the anterior cortex) made by the surgeon for proximal tunnel placement are revealed. Blue arrow in b points at the postoperative bone bridge (red line) between transosseous sutures (BBS). Red arrow in b points at the postoperative bone bridge (orange line) between tunnels (BBT)

Fig. 5

Sagittal, frontal and axial CT section of patella #10 postoperatively. Tunnel placement in a and b appears adequate. However, in c, two attempts made by the surgeon for proximal tunnel placement are revealed. The surgeon was probably unable to create a correct tunnel between previous attempted drillings. Blue arrow in b points at the postoperative bone bridge (red line) between transosseous sutures (BBS). Red arrow in b points at the postoperative bone bridge (orange line) between tunnels (BBT)