Suture Biomechanics and Static Facial Suspension（5）
COMMENT
Load-bearing ability and elongation with applied force are properties of critical importance to functional outcome in SFS. A lower load to failure increases the likelihood of suture breakage during the early postoperative period. We have clinically observed early breakage of 3-0 polypropylene suture on several occasions. Breaks have occurred most often along the Oral commissure vector. The mean maximum load to failure for 3-0 polypropylene, the suture we had previously chosen most frequently for SFS, was approximately 30 N. The alternative suture materials PBCP and PIP, which were evaluated in this study, tolerated significantly larger loads before failure in all suture sizes. If selected for SFS, these materials hypothetically would be less likely to fail in the early postoperative period.
Based on the load to failure of 3-0 polypropylene in this study and our clinical experience with failure at the oral commissure when using this material, one may conclude that it is not uncommon to place a load of 30 N along this vector in vivo. Elongation data were collected at 20 and 30 N for this reason; we believe that these are loads frequently encountered clinically on individual sutures in SFS. Substantial Elongation of the suture material will compromise functional and aesthetic outcomes. The elongation of size 3-0 polypropylene suture was greater than 25% (or >1 cm) when a force of 30 N was applied. Elongation of size 3-0 PBCP and PIP was around 10%. Size 0 PBCP and PIP elongate only about 5%. The shorter gauge length used in vivo would decrease the amplitude of elongation witnessed clinically, but these results unequivocally demonstrate that polypropylene suture loaded with 20 to 30 N will consistently elongate 2 to 3 times more than either PBCP or PIP. Likewise, stiffness values were significantly greater for PBCP and PIP than for polypropylene. In a study of sutures used in tendon repair, Lawrence and Davis6 also concluded that Polyfilament braided polyester sutures such as PBCP provide greater stiffness than polypropylene did. Static facial suspension may not maintain oral competence and facial appearance when there is inadequate stiffness and significant stretch occurs. Increased early elongation seems to forecast a propensity for greater creep over time as well, but this study cannot definitively prove such a claim.
Previous studies by Morgan et al4 and Sclafani et al5 have examined the biomechanical properties of human acellular dermis (AlloDerm) and expanded PTFE (Gore-Tex) as used in SFS. Data obtained by Morgan et al for the load to failure and stiffness of these materials is shown in Table 4. Stiffness is inversely proportional to sample gauge length, and stiffness data from the earlier study4 were appropriately halved in this table to compensate for the longer gauge length used in the present study. These stiffness values were then used to calculate the elongation that would occur in these materials with a load of 30 N. The mean load to failure of either of these materials initially appears to be much greater than that for any of the suture groups. However, placing 3 sutures allows for load sharing along 3 vectors. Load sharing would be equal if the sutures were placed in parallel, essentially tripling the load to failure, but, because the sutures have vectors acting at different angles, the load is not evenly distributed. This is determined in part by the surgeon, who must select the appropriate amount of tension for each suture before tying and securing it at the lateral rim. In our experience, more tension is required along the Oral commissure vector to prevent Oral ptosis.
Table 4. Properties of Human Acellular Dermis and Expanded PTFE*
Figure 4A demonstrates the suture placement and vectors used in the SFS technique. There is an approximately 90° angle between lines drawn from the lateral orbital rim to the nasal ala and along the nasolabial fold. This right angle can be used to calculate the force acting along any vector originating from the rim and passing through the nasolabial fold as the hypotenuse of a right triangle. Using this supposition, potential force distribution can be calculated between 3 sutures placed at the aforementioned sites along the nasolabial fold in SFS as in Figure 4A. In Figure 4B and C, the larger blue arrow represents a single vector along which a human acellular dermis or expanded PTFE sling might act. This same load is distributed along 3 vectors at different angles when a suture sling is used. We considered the clinical load that we believe is placed on each suture in vivo to estimate distribution between and to calculate the load placed on the 3 smaller vectors, represented by the red arrows. The sum of the x and y components of these 3 smaller vectors is equal to the x and y components of the larger vector, but the sum of the 3 smaller vectors themselves does not equal that of the larger vector because of the differences in angles at which multivector (eg, suture) and single-vector (eg, human acellular dermis or expanded PTFE) slings act (ie, the sum of 17, 30, and 26 is 73, not 70). The loads in Figure 4B represent performance that could reasonably be expected when using size 3-0 polypropylene suture, based on our data. The total load borne by the 3 sutures is greater than what would be expected from a single suture but still does not compare favorably with human acellular dermis and expanded PTFE. Figure 4C demonstrates the hypothetical load sharing when using size 0 PBCP or PIP.
Figure 4. Sutures are placed along 3 vectors in our static facial suspension technique.
A, Pledgets are shown at 3 sites along the nasolabial fold: the nasal ala, midfold, and oral commissure. There is a right angle created by lines drawn from the lateral rim to the nasal ala and along the nasolabial fold. Potential force distribution can be calculated using right triangles created by vectors passing through the nasolabial fold.
B, If a total load of 70 N is present, this load could be distributed as shown along vectors representing each of the 3 sutures. This depicts performance that could be expected using size 3-0 polypropylene suture (Prolene; Johnson & Johnson, New Brunswick, NJ).
C, A total load of 150 N could be borne with load sharing when using size 0 polybutilate-coated braided polyester (Ethibond Excel; Johnson & Johnson) or polyester impregnated with polytetrafluoroethylene (Tevdek; Teleflex, Inc, Limerick, Pa). Such performance is comparable to that of a single piece of human acellular dermis (AlloDerm; LifeCell Corporation, Branchburg, NJ) or expanded polytetrafluoroethylene (Gore-Tex; W. L. Gore & Associates, Flagstaff, Ariz).
The load to failure of both human acellular dermis and expanded PTFE in the previous study by Morgan et al4 exceeded 100 N. Size 0 PBCP or PIP could offer similar performance, with total load-bearing capacity exceeding 150 N. A limitation of any comparison between suture and materials such as human acellular dermis and expanded PTFE is that, unlike suture, different widths of other materials can be tested and wider samples may increase load-bearing capacity. Morgan et al used 15-mm-wide samples of human acellular dermis and expanded PTFE. The sheets used for SFS are typically wider than 15 mm, and the testing of sheets equaling the average width used clinically might be more useful for comparison with suture. The gauge length in the earlier study was 22 mm, which we compensated for as described in the “Methods” section. Once stiffness is adjusted for this gauge length differential, human acellular dermis has stiffness similar to sizes 0 and 2-0 PBCP and PIP. Expanded PTFE is less stiff and its performance is more similar to that of polypropylene.
We tested no biological materials in our study, but the study by Morgan et al emphasized the differences in load to failure between synthetic and biological materials. Table 2 demonstrates that human acellular dermis, a biological material, failed across a broad range of load values. Conversely, expanded PTFE, a synthetic material, failed across a narrow range.4 The synthetic suture materials we tested also performed very consistently, failing over a narrow range. A study by Choe et al7 also demonstrated this increased variability with human acellular dermis. The inconsistent performance of human acellular dermis occurs because the dermis must be harvested, which results in irregularities and weaker areas throughout most pieces. Expanded PTFE provides consistent performance but has been associated with frequent wound complications and extrusion in irradiated patients. In our hands, suture provides the consistent performance of a synthetic material without the wound complications seen with expanded PTFE.
Late postoperative failure of SFS has more varied causes than does early postoperative failure. Breakage may still occur but is probably more related to permanent structural alterations in a material over time than to initial load-to-failure capacity. A definite limitation to our study is that load to failure does not estimate how suture will perform over time, although this is an area of ongoing study in our laboratory. Late failure may also occur because there is sawing of distal tissues subjected to the continual suspension force. A PTFE pledget is used to minimize this problem; however, we have witnessed 2 cases in which both the pledget and the suture loop have remained intact but still dissected superiorly through the subcutaneous tissue. Biomechanical suture properties have no bearing on this process, and revision procedures have been required in these cases. Creep may also result in late failure of SFS with suture. A material's initial stiffness and elongation may or may not predict the propensity for and degree of creep that will occur over time.
Knowledge of the load to failure, stiffness, and elongation characteristics of different materials may assist the facial plastic surgeon in predicting the likelihood of SFS success or failure. Polypropylene suture performed poorly in all of these areas compared with PBCP and PIP. Many biomechanical properties of polypropylene are not ideal for SFS with suture. Higher load-bearing capacity could reduce the incidence of suture breakage. Decreased need for initial overcorrection to compensate for suture elongation and stretch would allow the surgeon to produce more predictable and consistent results. The biomechanics of PBCP and PIP sutures also compare favorably with human acellular dermis and expanded PTFE.
The differences between PBCP and PIP, as they relate to properties that will affect SFS performance, seem to be minimal, and either material seems to offer an excellent alternative to polypropylene for this application. A small number of patients in our practice have already undergone SFS using PBCP with good results. An added benefit has been that the braided PBCP is more malleable and less palpable over the lateral orbital rim than polypropylene is. We propose the adoption of heavy polyfilament braided-polyester sutures such as PBCP or PIP when a static suspension procedure is indicated for rehabilitation of patients with facial paralysis.
AUTHOR INFORMATION
Correspondence : Clinton D. Humphrey, MD, Department of Otolaryngology–Head and Neck Surgery, The University of Kansas Medical Center, Kansas City, KS 66160 (chumphrey@kumc.edu).
Accepted for Publication: January 11, 2007.
Author Contributions :
Study concept and design : Humphrey, Tsue, and Kriet. Acquisition of data: Humphrey and McIff.
Analysis and interpretation of data : Humphrey, McIff, Sykes, and Kriet. Drafting of the manuscript: Humphrey, Sykes, Tsue, and Kriet.
Critical revision of the manuscript for important intellectual content : Humphrey, McIff, Tsue, and Kriet.
Statistical analysis : Humphrey and Sykes.
Administrative, technical, and material support : McIff.
Study supervision : McIff, Tsue, and Kriet.
Financial Disclosure : None reported.
REFERENCES
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