Paper 09: Postage-Stamp Glenoid Fracture after Bankart Repair. How Many Anchors are Safe? - A Biomechanical Analysis

OBJECTIVES: Fractures of anterior glenoid rim, aka “Postage-Stamp” fractures, are potential complications of arthroscopic Bankart repairs that occur when a fracture line propagates through previous anchor sites producing a serrated edge. (Fig.1: A and J) Placing multiple anchors adjacent to glenoid rim in attempt to recreate the labral bumper was suggested to reduce the risk of recurrent dislocation after surgery, ([Boileau et al 2006]) as well as to create a stress riser that may predispose to fracture if another traumatic event may occur. This controlled laboratory study investigated the effect of increased number holes, and different types and sizes of suture anchors on the load necessary to create Postage-Stamp fractures. Our objective was to establish a safe number of anchors that could be inserted in the anterior glenoid rim without incurring in a substantially high risk of fracture. HYPOTHESIS: Increasing the number of holes and anchors and their size would decrease forces necessary to break the anterior glenoid rim. METHODS: We tested 46 synthetic scapulae with similar compressive strength and elastic modulus of human glenoid (4th gen. composite scapulae, Sawbones, Pacific Research Laboratories, USA) in a servohydraulic apparatus (Fig.1: B and H) following a previous published biomechanical model. ([Farmer et al2014]) A guide ensured holes were drilled and anchors were inserted in the same exact angle, depth and location for every specimen. (Fig.1: C to F) A metallic humeral head applied force to the anterior glenoid rim at 1mm/s until fracture occurred. (Fig. 1: G to K) Load-to-fracture of intact glenoid was compared to groups of drilling anchor holes (# 3,4,5,6 and 7) of different diameters (1.6mm and 3mm), and groups with anchors of different sizes and types (1.6mm all-suture and 3mm core anchors).(Fig.1: I to K) One-way ANOVA followed by Turkey post-hoc test compared groups to determine the number of holes necessary to weaken glenoids below 70% of intact value with a p value <0.005. RESULTS: Intact glenoid mean load-to-fracture was 1,238 ± 74N. Drilling 3 to 7 holes of 1.6mm-diameter linearly reduced load to 93%, 89%, 74%, 56% and 52% of intact value respectively, while 3.0mm drill holes reduced load to 87%, 65%, 51%, and 40% respectively.(Fig.2) Directly comparing drill role sizes there were significant differences on the 4, 5 and 7 holes groups (p=0.045, 0.032 and 0.015, respectively), so that a glenoid could safely sustain up to 5 holes of 1.6mm, but no more than 3 holes of 3.0mm-diameter.(Fig.3) Inserting 1.6mm “all-suture” anchors in the 1.6mm-diameter holes did not change the load-to-fracture on the 4 or 5 holes groups (Fig.4A), indicating that the number of drill holes was the main determinant of anterior glenoid rim strength, so that the all-suture anchors did not act as stress risers. Thus, up to 5 “all-suture” 1.6mm anchor could be safely used on a Bankart repair. The 3.0mm “core” anchors increased the load-to-fracture when compared to the group with the same number of holes of the same diameter size. Despite 4 holes of 3.0mm-diameter decreased the load-to-fracture bellow the safety line of 70% of intact value, the group with 4 anchors of 3.0mm-diameter “core” anchors increased load-to-fracture to 85% of intact value (p=0.033).(Fig.4B) This stress shield effect of the 3.0mm core anchor was not noticed on the 5 anchors group, in which the overall strength of the construction was bellow the 70% of intact safety line. CONCLUSIONS: Our data call in that up to four 3.0mm “core” anchors or five 1.6mm-diameter “all-suture” anchors could be safely inserted in the anterior glenoid rim without incurring in increased risk of Postage-Stamp fracture. One should prefer small diameter anchors if more than 4 anchors were intended on a Bankart repair. This study contributed to important data about number, size and type of anchors that the anterior glenoid rim could safely sustain in order to avoid postage-stamp fracture in case a new traumatic dislocation episode occur after a Bankart repair, which is of clinical interest in high risk population preoperative planning and selection of implant size, type and number.