Traction endurance biomechanical study of metallic suture anchors at different insertion angles

Estudo biomecânico de resistência à tração de âncoras metálicas de sutura em diferentes ângulos de inserção

Abstracts

O objetivo deste trabalho é verificar se o ângulo de inserção das âncoras de sutura interfere na sua resistência quando submetidas à uma força de tração constante. Foram realizados 20 ensaios com âncoras metálicas de sutura inseridas em 4 angulações diferentes (30°, 45°, 60°, 90°), em osso de cadáver humano (fêmur distal) e mais 20 ensaios em osso artificial (SawboneTM). Os testes foram realizados na Máquina Universal de Ensaios modelo Kratos, sendo as âncoras tracionadas a 90° em relação ao seu plano de inserção na superfície óssea. Os valores encontrados no osso de cadáver humano não foram estatisticamente significantes, ou seja, o ângulo de inserção não influiu na resistência à tração. No osso artificial verificou-se que houve maior resistência na inserção a 90o, com relevância estatística em relação aos demais ângulos testados.

Âncoras; Biomecânica; Resistência à tração

ORIGINAL ARTICLE

Traction endurance biomechanical study of metallic suture anchors at different insertion angles

Flávia Namie Azato ^I ; André Toraso Yamasaki ^I ; Fábio Sucomine ^I ; Arnaldo Amado Ferreira Neto ^II ; Américo Zoppi Filho ^III ; Eduardo Benegas ^IV ; Alexandre Pagotto Pacheco ^IV ; Raul Bolliger Neto ^V ; César Augusto Martins Pereira ^VI

^IEx-resident

^IIAssistant Doctor at Shoulder and Elbow Group, Mentor of the work

^IIIAssistant Doctor and Head of Shoulder and Elbow Group

^IVAssistant Doctor of Shoulder and Elbow Group

^VAssistant Doctor and vice-responsible for the Biomechanics Laboratory LIM 41. Co-mentor of the work

^VIHealth Technologist and Researcher of the Biomechanics Laboratory LIM 41. Co-mentor of the work

^{Correspondence}

SUMMARY

The suture anchors' insertion angle and its traction resistance are the main subjects of this study.

Twenty trials were realized using threaded suture anchors in four diferents angulations (30º /45º /60º /90º) in human bone (distal femur) and another twenty trials in artificial bone (Sawbone^TM). The anchors were pulled out being tractioned uprightly from its bone surface by a Kratos Universal test machine.

The human bone results found no relation between the main subjects of this study, so whithout statistical value. On the other hand at the artificial bone the insertion angle of 90º beared more traction, being statistically significant compared to the other angles.

Key words: Anchors; Biomechanics; Pullout strenght.

INTRODUCTION

Suture anchors are increasingly being used due to technical simplicity they add to Orthopedic Surgery, both open as arthroscopic, allowing surgery time reduction and a consequent lower morbidity of the surgery^(7,8,15,17)

As their use increased, mainly in shoulder surgery (rotator cuff and gleno-humeral instability correction), the loosening of these implants are possibly occurring complications in early more aggressive postoperative conduct. Several papers present biomechanical studies related to endurance to pullout strength of tendon sutures or soft tissues, with the use of bone fixation anchors (either or not screwed)⁽¹⁶⁾ comparing them to bone transfixing ones^(1,5,11,14). However few papers were found⁽²⁰⁾ related to the anchor endurance in relation to the bony surface itself (without a sutured soft tissue). It was also not found any paper measuring the endurance of these anchors at different insertion angles.

Regarding the best insertion angle of the anchor in rotator cuff sutures, Burkhart ⁽⁴⁾ suggests that, for supraspinatus tendon suture, the ideal anchor insertion at the great tubercle would be 45º in relation to humeral shaft. In this paper, the author calls this the Deadman angle, however does not mention the pullout endurance of different angle insertions.

Considering that during the surgery is often difficult to precisely determine the anchor insertion angle, in this paper we aimed to compare the suture anchors pullout endurance when inserted at different angles.

MATERIAL E METHODS

A pair of knees from a single cadaver, male, 51 years old, with no previous bone surgery or osteometabolic disease. The specimens were kept frozen at –20ºC to the moment of the assay. After defrosted at room temperature, soft tissues were removed and the knees disarticulated to obtain two single pieces of dissected femurs, right and left.

Two artificial bone (Sawbone^TM) units model 1414-5 with cancellous bone characteristics with the following specifications: density, 20 pcf (pounds per cubic foot) and one elasticity module of 260 megapascal, according to supplier specification.

Twenty screwed suture anchors were used, made of stainless steel with the following specifications according to American Society for Tests of Material (ASTM): 4 mm diameter, 2.5 mm core, 12 mm high, screw pace 2mm (Figure 1).

The assays were performed in an Universal Kratos K 5002 Test Machine (with load cell of 100 Kgf) and the pullout force measured in Newton (Figure 2). Were also used interlaced steel wires (0.9 mm) for anchor pullout and a digital caliper and goniometer attached to the fixator, leading to a more precise assay⁽²⁰⁾.

Biomechanical assays:

Twenty assays in cadaver bone and twenty in artificial bone were performed.

Insertion sites were placed in metaphyseal region of distal femur with a digital caliper 15 mm distance between insertion sites, allowing the bone injury caused in a point not to influence next inserted anchor endurance^(2,5).

Studied insertion angles were 90º, 60º, 45º and 30°, and pullout force always applied at 90° regarding the insertion plane (Figure 3).

The bone was initially fixated to a fixator coupled to a goniometer. An electrical drill mounted with a 2.5 mm Kirschner wire was also coupled to the test machine, perpendicular to the floor. Due to bone surface irregularity, insertion sites were performed by means of the set "drill-test machine" as follows: a small tripod was coupled to a Kirschner wire so that it was kept perpendicular to the bone surface; from that, the "tripod-Kirschner" set was aligned to the set "driller-test machine" Kirschner wire, keeping them parallel. In this manner we got a plane that we called "zero mark". From this mark, the insertion angles were obtained by twisting the "bone-fixator" set, and the angles measured by means of the goniometer coupled to this set. The drillings were performed to reach 13 mm deep for anchor insertion according to the supplier specifications. The drill was changed by the load cell, returning the fixator to the zero point, so that the pullout would be perpendicular to the bone plane. The tested anchor was fixated to the load cell by means of an interlaced steel wire (0.9 mm) without causing tension to the first one.

Then, pullout was started with a 20 mm/min speed up to the anchor was totally removed (Figures 4, 5 and 6).

The same method was used for the artificial bone.

The following parameters were evaluated:

1) Proportionality Limit Strength (FLP): means the strength in the limit of the linearity that the complete suture anchor set endured before plastic deformity occurred, that is, after FLP was overcome the system had a plastic deformation, not returning to its original features, even after pullout is removed.

2) Maximal strength (Fmax): means the maximal force the system endured before being plucked out from the material, that is, after Fmax was overcome the tension decreases even with the system displacement.

Statistical study:

In cadaver bone, data retrieved for each insertion angle were evaluated according to Kruskal-Wallis statistical method in regard to FLP and Fmax with a p < 0.05 significance level.

In artificial bone it was used the variance analysis (ANOVA) for FLP and Fmax, and Tukey Statistic Comparison Test for evaluating the difference of the results between the different insertion angles, paired, with a significance level of p < 0.05.

RESULTS

The studied variables were the different suture anchor insertion angles: 90º, 60º, 45º and 30º, regarding FLP and Fmax. The results regarding FLP and Fmax in different insertion angles in cadaver bone are in (Tables 1 and 2).

In cadaver bone, significant differences between the different insertion angles were not found. Kruskal-Wallis test was used for statistical analysis and found p > 0.05 for FLP and Fmax.

The results regarding FLP and Fmax for different insertion angles in artificial bone are in (Tables 3 and 4).

In artificial bone it was found a statistically significant difference between the different insertion angles (p < 0.05) both regarding FLP and Fmax. ANOVA test was used for analyzing the data.

By means of Tukey Multiple Comparison Test, it was compared the statistical difference between the different insertion angle pairs in regard to FLP and Fmax for results in synthetic material (Table 5).

It was found that for FLP all angles but 30º and 45º presented statistical differences, that is, 90º insertion has a higher FLP than 60°, 45° and 30°. Insertion at 60º has a higher FLP than at 45º or 30º, however there is not a significant difference between insertion at 45º and 30º in regard to FLP.

Regarding Fmax, all values were statistically different, decreasing as insertion angle decreased (Graphic 1).

DISCUSSION

As it is an easy and practical method, suture anchors are being increasingly used in orthopedic surgeries both open^(8,15) and arthroscopic⁽³⁾, allowing the reduction of surgery time and, consequently, reducing postoperative morbidity^(7,8,15,17).

These are frequently used in capsular-labrum-ligamental shoulder instabilities, as well in reinsertion of supraspinous tendon tears in rotator cuff injuries^(9,10,13,19). However, in these situations, problems were found in regard to the pullout efforts exerted by this tendon, mainly regarding anchor insertion sites – the cancellous bone at great tuberosity region – and also in regard to the best insertion angle⁽⁴⁾.

Burkhart⁽⁴⁾ describes that, for suture of this tendon, the ideal anchor insertion angle in the great tuberosity would be of 45º in relation to the humeral shaft. In this paper he calls the insertion angle as "Deadman angle", however he does not mention the endurance of other insertion angles. As during arthroscopic surgery it is difficult to exactly determine if the insertion is performed at 45º, and as change in the insertion angle could interfere with pullout endurance, we aimed to perform the tests with anchors inserted at different angles and to evaluate these changes.

For a reproduction that is closer to the practical use of these implants, several biomechanical works used cadaver bony tissue, which in shoulder surgeries more frequently use proximal humerus and glenoid ^{(12,17,18,20)}. However in our work we did not use proximal humerus due to it would be necessary a large number of specimens, making the sample too much heterogeneous in regard to bony quality (osteoporosis)^(6,8). As great tuberosity surface is narrow, it would be possible to perform only two assays with anchors inserted in this region, in order to not having interference in local bone endurance ⁽²⁾. This way, we performed the assays with a pair of cadaver distal femur metaphyseal region, and with artificial bone, aiming to use two different material and obtain more homogeneous insertion surfaces allowing a larger number of assays⁽⁶⁾.

It was observed that during the anchor introduction there was a resistance variation of the different insertion places in femur metaphyseal bone that was not observed in artificial bone.

It was found that both in cadaver and in artificial bone, Fmax values decreased as the anchor insertion angle decreased.

However, in statistical analysis of cadaver bone assays, results comparing Fmax and also FLP of different insertion angles were not statistically significant, differently from the artificial bone. An explanation for that is probably due to variation of the texture of the different material used (distal metaphyseal femoral bone and artificial bone) also found by the researcher during the anchors insertion.

When comparing the different insertion angles to each other, it was found that Fmax of anchors inserted at 90º was higher than the other angles, that is, they were more resistant to pullout forces. We attribute this to the manner the pullout force was applied, perpendicular to the bone surface plane. In this way, in the case of anchors inserted at 90º the desinsertion force (TA) was the pullout force itself (T). Yet with anchors inserted at 30º, 45º and 60º, pullout force (T) decomposed into two forces (TA and TN), which together twisted the anchor to a more vertical position (Figure 7). We observed during the assays that before reaching its Fmax the anchor moved from its inclined position up to reach a 90º position, being after plucked out of the bone. We also verified that the bone over the anchor (hachured area) was tear by the anchor itself.

We emphasize that, even though with a small number of specimens, the quality of the bone tissue may interfere in pullout endurance⁽¹²⁾, additionally to the insertion angle before mentioned.

Nevertheless the assays performed in this work were of static resistance, which not exactly reproduces the requests to these implants "in vivo", we could wonder that some situations in this work are similar to those found in surgical practice. It is know that the pullout force exerted by the supraspinous tendon over the anchor has an angle close to 45º in relation to the surface of the great tuberosity. However, this angle and its relationship to the anchor change as humeral abduction takes place, changing the pullout force exerted over this implant. So, the lower endurance related to osteoporotic bone may influence on these material loosening.

It is worthy to stress that the ideal for this work would be the use of several specimens of proximal humerus with the same or similar bony quality, somehow reproducing situations close to reality.

CONCLUSIONS

1) Suture anchors inserted at 90° have more pullout endurance when compared to other insertion angles in artificial bone.

2) The quality of the material at the insertion site may interfere in the endurance of the anchors to pullout, independently of the insertion angle.

Trabalho recebido em 05/08/2002

aprovado em 27/11/2002

Work performed at the Departamento de Ortopedia e Traumatologia da Faculdade de Medicina da Universidade de São Paulo, FMUSP, SP

• 1. Amis AA, The strenght of artificial ligament anchorages: a comparative experimental study. J Bone Joint Surg Br 70:397-403, 1998.
• 2. Barber FA, Cawley P, Prudich JF. Suture anchor failure strength: an in vivo study. Arthroscopy 9:647-652, 1993.
• 3. Barber FA., Herbert MA, Click JN. Suture anchor strength revisited. Arthroscopy 12:32-38, 1996.
• 4. Burkhart SS. The deadman theory of suture anchors: observations along a south texas fence line. Arthroscopy 11:119-123, 1995.
• 5. Burkhart SS, Pagŕn JL, Wirth MA, Athanasiou K.A. Cyclic loading of anchor-based rotator cuff repairs: confirmation of the tension overload phenomenon and comparison of suture anchor fixation with transosseous fixation. Arthroscopy 13:720-724, 1997.
• 6. Carpenter JE, Fish DN, Huston LJ, Goldstein SA. Pull-out strength of five suture anchors. Arthroscopy 9:109-113, 1993.
• 7. Craft DV, Moseley JB, Cawley, PW, Noble PC. Fixation strenght of rotator cuff repairs with suture anchors and the transosseous suture technique. J Shoulder Elbow Surg 5:32-40, 1996.
• 8. Fennel CW, Ballard JM, Pflaster DS, Adkins RH. Comparative evaluation of bone suture anchor to bone tunnel fixation of tibialis anterior tendon in cadaveric cuboid bone: a biomechanical investigation. Foot Ankle Int 16:641-645, 1995.
• 9. France EP, Paulos LE, Hanrner CD, Straight CB. Biomechanical evaluation of rotator cuff fixation methods. Am J Sports Med, 17:176-181, 1989.
• 10. Gerber C, Schneeberger AG, Beck M, Schlegel U. Mechanical strength of repairs of the rotator cuff. J Bone Joint Surg Br 76:371-80, 1994.
• 11. Goble EM, Somers WK, Clark R, Olsen RE. The development of suture anchors for use in soft tissue fixation to bone. Am J Sports Med 22:236-239, 1994.
• 12. Hecker AT, Shea M, Hayhurst JO, Myers ER, Meeks LW, Hayes WC. Pull-out strenght of suture anchors for rotator cuff and Bankart lesion repairs. Am J Sports Med 21:874-879, 1993.
• 13. Matsen FA, Arntz CT. Rotator cuff tendon failure. In: Rockwood C.A., Matsen F.A., eds. The shoulder. Philadelphia: Saunders, 1990. p.671-673.
• 14. Reed SC, Glossop N. Full-thickness rotator cuff tears. A biomechanical comparison of suture versus bone anchor techniques. Am J Sports Med 24:46-48, 1996.
• 15. Rehak DC, Sotereanos DG, Bowman MW, Herndon JH, Pittsburgh PA. The mitek bone anchor: application to the hand, wrist and elbow. J Hand Surg Am 19:853-860, 1994.
• 16. Robertson DB, Daniel DM, Biden E. Soft tissue fixation to bone. Am J Sports Med 14:398-403, 1986.
• 17. Rossouw DJ, McElroy BJ, Amis AA, Emery RJ. A biomechanical evaluation of suture anchors in repair of the rotator cuff. J Bone Joint Surg Br 79:458-461, 1997.
• 18. Sasaki SU, Stuginsky RM, Mattar Jr R, et al. Estudo biomecânico comparativo da resistęncia ŕ traçăo entre dois tipos diferentes de mini-âncora de sutura. Rev Bras Ortop 35:231-234, 2000.
• 19. Sward L, Hughes JS, Amis A, Wallace WA. The strength of surgical repairs of the rotator cuff: a biomechanical study on cadavers. J Bone Joint Surg Br 74:585-588, 1992.
• 20. Wetzler MJ, Bartolozzi AR, Gillespie MJ, et al. Fatigue properties of suture anchors in anterior shoulder reconstructions: mitek GII. Arthroscopy 12:687-693, 1996.

Publication Dates

• Publication in this collection
  16 Apr 2003
• Date of issue
  Jan 2003

History

• Accepted
  27 Nov 2002
• Received
  05 Aug 2002