Single versus double row suture anchor fixation for greater tuberosity fractures – a biomechanical study

Isolated fractures of the greater tuberosity represent a common injury accounting for up to 1/5th of all proximal humeral fractures [1, 10]. Due to the limited dimension of the subacromial space even small residual superior displacement may cause clinical impairment [2, 9]. It has been shown, that conservative treatment for dislocated greater tuberosity fractures is associated with pain and impaired range of motion (ROM) [8]. Therefore, especially in patients with high functional demands surgical treatment is recommended if superior dislocation of the greater tuberosity reaches 3-5 mm of displacement [4, 9]. Gerber and Warner described that malunion is one of the most common complications following surgical treatment of proximal humeral fractures [9]. As a general rule, it is well known that bony healing depends critically on anatomical reposition and rigid retention [31]. Besides, mechanical stability is essential to allow early functional rehabilitation in order to achieve satisfying clinical results [15]. Arthroscopic treatment of greater tuberosity fractures is an established method [12, 28, 29]. Previous biomechanical studies demonstrated a secure retention and reposition using suture anchors [3]. However, none of theses studies investigated on the influence of the postero-superior rotator cuff (M. infraspinatus (ISP) and M. supraspinatus (SSP)) forces on the stability of greater tuberosity fractures that were treated arthroscopically using suture anchor techniques. Further, to the knowledge of the authors, there is no biomechanical data comparing single vs. double-row anchor fixation.

The aim of this biomechanical study was to compare knotless suture anchor reconstruction in greater tuberosity fractures using a single- vs. double-row fixation. Therefore, a biomechanical human cadaveric in vitro model was established, that incorporated forces of the postero-superior rotator cuff on the greater tuberosity.

The primary hypothesis of this study was, that double-row fixation confers greater load-to-failure strength and less secondary displacement under cyclic loading compared to single-row fixation of the greater tuberosity.

This biomechanical study was approved by the institutional review board of the Technical University Munich, Munich, Germany.

There were no significant differences between groups concerning BMD, morphology, and age (n.s.).

Regarding the mean cyclic load to failure (LtF) of suture anchor reconstruction in standardized fractures of the greater tuberosity, there were higher values in the DR group (649 N; ± 176) compared to the SR group (490 N; ± 145). (Fig. 5).

In the SR group the mean number of cycles to failure were 10,782 ± 4203 whereas construct failure could be observed in the DR group at 14317 ±4450 cycles in average.

During the testing it was recognized that a 5 mm displacement – as defined in prior studies [3, 4, 9, 17, 25] - could only be achieved in one fourth of all specimen before construct failure. Thus, it was preferred to measure the applied force (N) at 1 mm displacement to receive objective and comparable results between the groups. In the SR group the mean applied load for 1 mm displacement was 276.7 N ± 46.2 whereas in the DR group an average load of 259.5 N ± 61.8 could be observed to achieve 1 mm displacement.

However, there was no difference regarding the loads for 1 mm displacement between single and double row fixation of greater tuberosity fractures (p = .65) (Fig. 6).

In this biomechanical in-vitro study of arthroscopic suture anchor fixation techniques for greater tuberosity fractures, there was no significant difference between single-row and double-row repair regarding load to failure and applied traction force for 1 mm fracture displacement. Thus the studies primary hypothesis must be rejected. To the best of our knowledge, this is the first study presenting biomechanical data on single- vs. double-row knotless suture anchor repair of greater tuberosity fractures.

Treatment modalities of greater tuberosity fractures include conservative treatment, open, minimally invasive percutaneous, and arthroscopic procedures [9, 11, 13, 16, 17, 24, 26, 27]. It has been shown that nonoperative management reveals disappointing results in case of displaced fractures of the greater tuberosity [26]. Thus surgical treatment is advised if superior displacement of ≥3-5 mm occurs to avoid malunion of the fractured greater tuberosity.

During the last years the focus has moved towards arthroscopic techniques [6, 12, 14, 16, 23, 24] due to the benefit of reduced pain, soft tissue damage and skin incision. Additionally arthroscopy allows to inspect the entire glenohumeral joint as well as to address concomitant pathologies in one session [20]. Besides, arthroscopic suture anchor repair using knotless implants may reduce operative time, simplifies the procedures and waives suture knots. One of the benefits of suture fixation is that it depends on the strength of the rotator cuff tendons themselves and not only on the bone quality.

Furthermore, this technique seems to be superior to other rigid methods like screws regarding load to failure and load to 5 mm displacement [17]. This represents an important recognition as fractures of the greater tuberosity are commonly comminuted and usually seen in young and active patients [1, 18]. Furthermore, in case of comminuted fragments of the greater tuberosity or in older patients with osteopenic bone [10] a wider contact surface of refixation could be helpful regarding healing and stability [30]. In addition, no additive surgery to remove osteosynthesis material is needed.

A gradual increase of greater tuberosity displacement that requires intervention – usually described as 3-5 mm displacement [3, 4, 9, 17, 25] – could not be seen in this study.

Neither in the single-row nor in the double-row group all specimens consequently reached these two greater tuberosity displacement landmarks before construct failure.

This could be explained by an increased stability of the fixation method compared to rigid systems like two cancellous screws and due to the fixation of the entire tendinous insertion of the postero-superior rotator cuff (SSP/ISP). To receive objective and comparable results between groups, the applied traction force (N) was analyzed at 1 mm displacement (277 N (SR) vs. 260 N (DR)). Consequently, no fixation technique (SR vs. DR) showed significant advantages over the other. In this study medial row of the DR technique was not tied as knotless suture tape systems were used. This may have affected the 1 mm displacement that occurred more easily in the double row repair than the single row repair.

In their biomechanical study Lin et al. [17] described a 3 and 5 mm displacement of the greater tuberosity in the double-row suture anchor group at 262.5 N and 370.3 N load as well as in the suture-bridge technique at 321 N and 398.5 N load. In the two-screw fixation group a 3 and 5 mm displacement was apparent at a load of 186.7 N and 249.2 N, respectively.

This shows that the applied traction forces for 1 mm displacement are similar to the loads described by Lin et al. [17] for 3 mm displacement.

The ultimate failure loads in the study of Lin et al. [17] were 480 N (Double-row), 493.3 N (Suture-bridge) and 340 N for the two-screw fixation. The present study showed comparable failure loads in the SR group (490 N) and even higher loads in the DR group (649 N).Regarding failure modes, a fracture of the humeral surgical neck was noted in 50% of both groups indicating that the construct itself was stronger than the native bone. Lin et al. [17] also described a fracture of the humeral surgical neck as common failure mode. They also found a dislocation of the anterior anchor in one specimen of the Suture Bridge group.

In the present study, dislocation of a suture anchor was noticed in only one specimen (16,6%) per group.

A rupture of the abrasion-resistant wire fixation occurred twice (33,3%) in each group at more than 650 N in our study. This high load of failure underlines the findings of very stabile fixation methods – single- and double row. However, both fixation constructs are expected to tolerate significantly higher loads than the maximal supraspinatus force of ~302 N [5]. This might be explained by the fixation method. In this study the common footprint of the postero-superior rotator cuff in line with the SSP/ISP force vector was fixed instead of the sole supraspinatus tendon as formerly described [3, 17]. This was performed due to the described interdigitation of infraspinatus (ISP) and supraspinatus (SSP) fibres [7] with overlapping of the fibres [21] at the humeral footprint. Furthermore an extend of the infraspinatus` footprint along most of the highest facet of the greater tuberosity [22] is reported [19].

In addition FibreTape instead of FibreWire was used for suture anchor material that could also be responsible for the different failure loads although Ishak et al. [11] reported on equivalent load capacity. Nevertheless single SR reconstruction is an important mode of fixation. However, it may result in over reduction of the bone fragment distally. The major advantage of the double row technique is that the bone fragment is fixed at the osteoarticular junction and avoids over reduction of the fragment.

In clinical practice significantly softer bone stock is sometimes seen medially to the fracture. If there are concerns regarding the stability of the medial row of the double row reconstruction, single row fixation represents a high quality alternative but with biomechanically reduced load to failure. One of the major advantages of this suture technique is that fixation depends on the strongest tissue in the area which is the rotator cuff tendon itself.

Besides, in case of comminuted fractures of the greater tuberosity or in osteopenic bone, this study could provide important information regarding suture anchor reconstruction that underlines the clinical relevance of our study.

In this biomechanical study arthroscopic single- and double-row suture anchor repair of isolated greater tuberosity fractures seem to be viable options for treatment. Therefore, in the setting of reduced bone quality and weak anchorage of the medial row anchors, the authors perform a single row repair in daily clinical practice.