Magnetic resonance imaging analysis of the bioabsorbable Milagro™ interference screw for graft fixation in anterior cruciate ligament reconstruction

Abstract

Ligament graft fixation with bioabsorbable interference screws is a standard procedure in cruciate ligament replacement. Previous screw designs may resorb incompletely, and can cause osteolysis and sterile cysts despite being implanted for several years. The aim of this study was to examine the in vivo degradation and biocompatibility of the new Milagro TM interference screw (Mitek, Norderstedt, Germany). The Milagro TM interference screw is made of 30% ß-TCP (TriCalcium phosphate) and 70% PLGA (Poly-lactic-co-glycolic acid). In the period between June 2005 and February 2006, 38 patients underwent graft fixation with Milagro TM screws in our hospital. Arthroscopic ACL reconstruction was performed using hamstring tendon grafts in all the patients. MR imaging was performed on 12 randomly selected patients out of the total of 38 at 3, 6 and 12 months after surgery. During the examination, the volume loss of the screw, tunnel enlargement, presence of osteolysis, fluid lines, edema and postoperative screw replacement by bone tissue were evaluated. There was no edema or signs of inflammation around the bone tunnels. At 3, 6 and 12 months, the tibial screws showed an average volume loss of 0, 8.1% (±7.9%) and 82.6% (±17.2%, P \ 0.05), respectively. The femoral screws showed volume losses of 2.5% (±2.1%), 31.3% (±21.6%) and 92.02% (±6.3%, P \ 0.05), respectively. The femoral tunnel enlargement was 47.4% (±43.8%) of the original bone tunnel volume after 12 months, and the mean tunnel volume of the tibial tunnel was -9.5% (±58.1%) compared to the original tunnel. Bone ingrowth was observed in all the patients. In conclusion, the resorption behaviour of the Milagro TM screw is closely linked to the graft healing process. The screws were rapidly resorbed after 6 months and, at 12 months, only the screw remnants were detectable. Moreover, the Milagro TM screw is biocompatible and osteoconductive, promoting bone ingrowth during resorption. Tunnel enlargement is not prevented in the first months but is reduced by bone ingrowth after 12 months.

How to evaluate bone tunnel widening after ACL reconstruction - a critical review

“…The percentages of the absolute values to the original drill bit size were calculated 1, 7, 8, 15, 26, 31-33, 37, 39, 41-43, 46, 47 . In contrast to CT, MRI promises not only structural information on the tunnels, but also allows evaluation of the ACL graft itself 3,9,10,17,18,20,22,38,44 . Major drawback is the limited assessment of osseous structures in MRI.…”

“…4 c) 9 . The absolute and relative values in relation to the drill bit were used 9 . Kim et al assed tibial tunnel cross-sectional area by T1-and T2-weighted axial images which were obtained perpendicular to the long axis of the tibial tunnel 1 cm below the articular surface, 1 cm above the distal tibial tunnel exit, and at the midpoint of the tunnel 20 .…”

“…Absolute and relative values of the tunnel cross-sectional area were reported 10 . Frosch et al measured the width of the bone tunnels perpendicular to the tunnel axis in the sagittal plane 9 . The widest diameter was measured in three equal parts and the cross-sectional area was calculated (Fig.…”

“…The widest diameter was measured in three equal parts and the cross-sectional area was calculated (Fig. 4 c) 9 . The absolute and relative values in relation to the drill bit were used 9 .…”

“…Hence, the interpretation of absolute bone tunnel size measurements using radiographs can be misleading. In the last decade magnetic resonance imaging (MRI) and computed tomography (CT) were increasingly used for evaluation of tunnel widening, as these promised higher accuracy and lower inter-or intra-observer variabilities 9,10,18,45 . Only few Authors have used bone scans, single-photon emission tomography (SPECT) or multi-modality imaging such as SPECT/CT to follow-up their patients after ACL reconstruction 11,13 .…”