Biomechanical and viscoelastic properties of skin,SMAS, and composite flaps as they pertain to rhytidectomy

Previous studies have focused on biomechanical and viscoelastic properties of the superficial musculoaponeurotic system (SMAS) flap and the skin flap lifted in traditional rhytidectomy procedures. The authors compared these two layers with the composite rhytidectomy flap to explain their clinical observations that the composite dissection allows greater tension and lateral pull to be placed on the facial and cervical flaps, with less long-term stress-relaxation and tissue creep. Eight fresh cadavers were dissected by elevating flaps on one side of the face and neck as skin and SMAS flaps and on the other side as a standard composite rhytidectomy flap. The tissue samples were tested for breaking strength, tissue tearing force, stress-relaxation, and tissue creep. For breaking strength, uniform samples were pulled at a rate of 1 inch per minute, and the stress required to rupture the tissues was measured. Tissue tearing force was measured by attaching a 3-0 suture to the tissues and pulling at the same rate as that used for breaking strength. The force required to tear the suture out of the tissues was then measured. Stress-relaxation was assessed by tensing the uniformly sized strips of tissue to 80 percent of their breaking strength, and the amount of tissue relaxation was measured at 1-minute intervals for a total of 5 minutes. This measurement is expressed as the percentage of tissue relaxation per minute. Tissue creep was assessed by using a 3-0 suture and calibrated pressure gauge attached to the facial flaps. The constant tension applied to the flaps was 80 percent of the tissue tearing force. The distance crept was measured in millimeters after 2 and 3 minutes of constant tension. Breaking strength measurements demonstrated significantly greater breaking strength of skin and composite flaps as compared with SMAS flaps (p < 0.05). No significant difference was noted between skin and composite flaps. However, tissue tearing force demonstrated that the composite flaps were able to withstand a significantly greater force as compared with both skin and SMAS flaps (p < 0.05). Stress-relaxation analysis revealed the skin flaps to have the highest degree of stress-relaxation over each of five 1-minute intervals. In contrast, the SMAS and composite flaps demonstrated a significantly lower degree of stress-relaxation over the five 1-minute intervals (p < 0.05). There was no difference noted between the SMAS flaps and composite flaps with regard to stress-relaxation. Tissue creep correlated with the stress-relaxation data. The skin flaps demonstrated the greatest degree of tissue creep, which was significantly greater than that noted for the SMAS flaps or composite flaps (p < 0.05). Comparison of facial flaps with cervical flaps revealed that cervical skin, SMAS, and composite flaps tolerated significantly greater tissue tearing forces and demonstrated significantly greater tissue creep as compared with facial skin, SMAS, and composite flaps (p < 0.05). These biomechanical studies on facial and cervical rhytidectomy flaps indicate that the skin and composite flaps are substantially stronger than the SMAS flap, allowing significantly greater tension to be applied for repositioning of the flap and surrounding subcutaneous tissues. The authors confirmed that the SMAS layer exhibits significantly less stress-relaxation and creep as compared with the skin flap, a property that has led aesthetic surgeons to incorporate the SMAS into the face lift procedure. On the basis of the authors' findings in this study, it seems that that composite flap, although composed of both the skin and SMAS, acquires the viscoelastic properties of the SMAS layer, demonstrating significantly less stress-relaxation and tissue creep as compared with the skin flap. This finding may play a role in maintaining long-term results after rhytidectomy. In addition, it is noteworthy that the cervical flaps, despite their increased strength, demonstrate significantly greater tissue creep as compared with facial flaps, suggesting earlier relaxation of the neck as compared with the face after rhytidectomy.     

A finite viscoelastic–plastic model for describing the uniaxial ratchetting of soft biological tissues

In this paper, a phenomenological constitutive model is constructed to describe the uniaxial ratchetting (i.e., the cyclic accumulation of inelastic deformation) of soft biological tissues in the framework of finite viscoelastic-plasticity. The model is derived from a polyconvex elastic free energy function and addresses the anisotropy of cyclic deformation of the tissues by means of structural tensors. Ratchetting is considered by the evolution of internal variables, and its time-dependence is described by introducing a pseudo-potential function. Accordingly, all the evolution equations are formulated from the dissipation inequality. In numerical examples, the uniaxial monotonic stress–strain responses and ratchetting of some soft biological tissues, such as porcine skin, coronary artery layers and human knee ligaments and tendons, are predicted by the proposed model in the range of finite deformation. It is seen that the predicted monotonic stress–strain responses and uniaxial ratchetting obtained at various loading rates and in various loading directions are in good agreement with the corresponding experimental results.     

Purse string-formed plication of the SMAS with fixation to the zygomatic bone

The superficial musculoaponeurotic system (SMAS) operation revolutionized face-lift procedures. The idea of having one direction of traction with suturing and fixation of SMAS to a stable structure is gaining popularity. The author's contribution is the fixation of the SMAS and the extension of the supraplatysmal plane to the zygomatic bone periosteum to achieve stable traction and fixation. This procedure has not been described before in medical literature. The purse string-formed plication of the SMAS is a procedure in which the soft tissue (SMAS, parotid fascia, and the extension of the supraplatysmal plane) is plicated and fixed to the periosteum of the zygomatic bone. This superficial operation avoids entering the deeper planes, which could result in severe complications. The procedure fills the cheeks and gives a more natural look than standard face lifts. A total of 250 patients underwent this procedure. The suspension achieved seemed to be more stable than some SMASectomy techniques. Complication rates and recovery times were low. The purse string-formed plication of the SMAS is a safe, quick, and simple procedure with effective results suitable for younger patients with good skin conditions.     

A constitutive formulation of vascular tissue mechanics including viscoelasticity and softening behaviour

Nearly all soft tissues, among which the vascular tissue is included, present a certain degree of viscoelastic response. This behaviour may be attributed in part to fluid transport within the solid matrix, and to the friction between its fluid and solid constituents. After being preconditioned, the tissue displays highly repetitive behaviour, so that it can be considered pseudo-elastic, that is, elastic but behaving differently in loading and unloading. Because of this reason, very few constitutive laws accounting for the viscoelastic behaviour of the tissue have been developed. Nevertheless, the consideration of this inelastic effect is of crucial importance in surgeries—like vascular angioplasty—where the mentioned preconditioning cannot be considered since non-physiological deformation is applied on the vessel which, in addition, can cause damage to the tissue. A new constitutive formulation considering the particular features of the vascular tissue, such as anisotropy, together with these two inelastic phenomena is presented here and used to fit experimental stress–stretch curves from simple tension loading–unloading tests and relaxation test on porcine and ovine vascular samples.     

Cigarette smoking and face lift: conservative versus wide undermining

The effects of cigarette smoking on the skin flaps of the face lift procedure are discussed. Reported elsewhere is a significant incidence of skin slough in smokers with use of wide undermining techniques. This complication is thought to be due to the vasoconstrictive effects of nicotine on the peripheral circulation. Our group has employed a conservative bilateral undermining technique in 407 face lifts. Of these, 32.4 percent were smokers and 67.6 percent were nonsmokers. No cases of skin slough were encountered. Our conservative undermining technique is briefly discussed. Among its advantages are shorter operative time, use of less local and/or general anesthesia, less intraoperative bleeding, adequate exposure for SMAS and platysmal surgery, and snugger skin closure without the risk of flap necrosis. As shown by our statistics, it is a safer procedure in smokers than the usually performed more radical procedure.     

The biomechanical effects of deep tissue support as related to brow and facelift procedures

The adverse effects of increased tension across a healing wound are well known. However, the effect of closing a wound in layers in order to decrease tension on the epidermis has been a source of controversy. It is hypothesized that deep tissue support decreases skin tension upon wound closure. In order to clarify this issue, a two-part study was designed to address the immediate effects of deep tissue support in vitro using fresh-frozen cadavers and in vivo on patients undergoing scheduled surgery. Closing skin tension was measured at standard reference points in coronal brow lift and rhytidectomy procedures performed with and without galeal closure and superficial musculoaponeurotic system (SMAS) procedures, respectively. Deep tissue support was found to significantly (p less than 0.05) decrease skin tension at the time of skin closure at standard reference points in coronal brow lift and rhytidectomy procedures performed on fresh-frozen cadavers. Similar significant (p less than 0.05) decreases in closing skin tension also were found in vivo in patients undergoing similar surgical procedures. Stress relaxation was not found to play a significant role in contributing to this immediate decrease in closing skin tension. It would appear, therefore, that deep tissue support, in the form of galeal closure and an SMAS procedure in coronal brow lift and rhytidectomy procedures, respectively, provides increased viscoelastic support, producing immediate significant decreases in closing skin tension in these procedures. The beneficial effects on wound healing, scar formation, tension-related trophic skin changes, and possible improved long-term results are discussed.     

Refinements in face lifting: enhanced facial contour using vicryl mesh incorporated into SMAS fixation

As we have gained experience with the extended superficial musculoaponeurotic system (SMAS) technique in face lifting, refinements in our procedure have led to increased consistency in results. The important factors that have led to our technical modifications include the following: (1) the significance of the retaining ligaments of the midface, which determine the degree of surgical dissection required for both skin and SMAS in rhytidectomy; (2) the changes in facial shape that occur with aging, secondary to the descent of facial fat; (3) the possibility of modifying facial shape through the repositioning of facial fat in an extended SMAS face lift; (4) the improved longevity of result to be obtained by incorporating Vicryl mesh into SMAS fixation; (5) the artistic nuances of incision design that help to minimize scar perceptibility. Understanding these factors enables surgeons to use the extended SMAS technique successfully with more challenging cases, enhancing facial appearance while minimizing signs that the patient has undergone a surgical procedure.     

Combined rhytidectomy and full-face laser resurfacing

A series of patients undergoing a combined face lifting procedure with simultaneous laser resurfacing is described. Although resurfacing is accepted as safe for deep-plane face lifts and forehead lifts, there are reports of preauricular skin loss with a standard face lift. In this series, 26 consecutive cases are described. A superficial musculoaponeurotic system (SMAS) face lift technique was used. A skin flap was developed no more than 3.0 cm from the preauricular area, and most of that was excised. Full-face laser resurfacing was done with the SilkTouch laser. An 8-mm square pattern was used at 16 to 18 W. Three to four passes were done. Care was taken, however, to do very superficial lasering of the periphery, especially over the small amount of undermined skin that remained. There was no preauricular skin loss. This series demonstrates the safety of combining laser resurfacing with the SMAS technique face lift in regard to preauricular skin loss.     

MASSIVE AND RECURRENT HERNIAS—Use of Dermal Grafts in Carrying Out Repair

Graft of dermis is clinically and experimentally superior to graft of whole skin for use as reinforcement at the site of repair of hernia, for gross cysts do not form and it fuses better with the surrounding tissues than does whole skin. Placing either dermis or skin under tension helps prevent cyst formation and aids fusion with surrounding tissues. Dermal grafts are less liable to infection than are whole skin grafts, either at the time of operation or later.A split-split flap dermatome technique is presented as a preferred technique for obtaining a dermal graft from the thigh in cases of ventral hernia in which the abdominal skin is stretched, attenuated and inelastic because of the massive size of the hernia. This technique was used in 27 cases of massive hernia. There was infection in one case of the 27 and subsequent healing was satisfactory. One patient died of spontaneous rupture of an intracranial carotid aneurysm. Hernia did not recur in any patient.A split-split free graft dermatome technique is presented for use in cases in which an elliptical segment of normal skin can be removed adjacent to an inguinal or thoracic incision for repair of a hernia or other use. This technique was used in seven cases of inguinal hernia and in one of diaphragmatic hernia with satisfactory results.     

A 5-year study of the transmalar subperiosteal midface lift with minimal skin and superficial musculoaponeurotic system dissection: a durable, natural-appearing lift with less surgery and recovery time

The transmalar subperiosteal midface lift is a simple, direct-approach procedure to be performed with a meloplasty. The entry into the midface is at the site of maximum suture tension, which allows for more elevation. The skin is elevated enough to expose the entry site, which is on the zygoma just cephalad to the origins of the zygomaticus muscles. Through a small hole at that site, a periosteal elevator is used for the midface dissection. This is a blind dissection, and the technique is described. The advantages of the technique are that there is (1) no lower-lid incision or risk of an ectropion, (2) a resultant tightening and elevation of the lower lid, (3) more elevation and durability because the zygomaticus muscle origins are elevated with the periosteum and are sutured to the very substantial deep temporal fascia, (4) a simple and fast procedure, and (5) no telltale sign of a face lift. Both the superficial musculoaponeurotic system (SMAS) and the skin are substantially elevated with the transmalar subperiosteal midface lift to the extent that they should be only minimally dissected. In the author's opinion, the extended dissection of the skin and/or the SMAS does not increase the amount of tissue lift and probably reduces it in most cases, considering that the goal is a natural look and not one that appears pulled or stretched. The skin is elevated only for exposure, and the SMAS is elevated only enough to create a preauricular SMAS-platysma flap to tighten the neck. With two fewer layers of dissection, there is significantly less postoperative swelling and recovery time. The article presents the technique, the results on 272 patients over a period of 5 years, and a discussion. No patients described have had secondary procedures such as lasers, so the transmalar subperiosteal midface lift can be evaluated on its own merit.     

Site specific inelasticity of arterial tissue

Understanding the mechanical behaviour of arterial tissue is vital to the development and analysis of medical devices targeting diseased vessels. During angioplasty and stenting, stress softening and permanent deformation of the vessel wall occur during implantation of the device, however little data exists on the inelastic behaviour of cardiovascular tissue and how this varies through the arterial tree. The aim of this study was to characterise the magnitude of stress softening and inelastic deformations due to loading throughout the arterial tree and to investigate the anisotropic inelastic behaviour of the tissue. Cyclic compression tests were used to investigate the differences in inelastic behaviour for carotid, aorta, femoral and coronary arteries harvested from 3-4 month old female pigs, while the anisotropic behaviour of aortic and carotid tissue was determined using cyclic tensile tests in the longitudinal and circumferential directions. The differences in inelastic behaviour were correlated to the ratio of collagen to elastin content of the arteries. It was found that larger inelastic deformations occurred in muscular arteries (coronary), which had a higher collagen to elastin ratio than elastic arteries (aorta), where the smallest inelastic deformations were observed. Lower magnitude inelastic deformations were observed in the circumferential tensile direction than in the longitudinal tensile direction or due to radial compression. This may be as a result of non-collagenous components in the artery becoming more easily damaged than the collagen fibres during loading. Stress softening was also found to be dependent on artery type. In the future, computational models should consider such site dependant, anisotropic inelastic behaviour in order to better predict the outcomes of interventional procedures such as angioplasty and stenting.     

Rhytidectomy and the nasolabial fold.

I describe a technical modification in the Skoog face lift procedure that releases the deep attachments of the SMAS to the muscles of facial expression for maximal mobility of the medial cheek yet elevates the cheek flap as a composite of skin, subcutaneous tissue, and SMAS to enhance skin perfusion. My results with the procedure in 100 patients are analyzed by using functional zones of the nasolabial fold corresponding to underlying musculature and a simple grading system based on preoperative and postoperative photographs. Marked improvement in the nasolabial fold was noted in over 80 percent of patients by 6 and 12 months postoperatively. This effect seemed to last up to 4 years.     

A Quantitative Study of Regional and Stage Specific Reaction of Xenopus laevisEmbryonic Tissues on Mechanical Load

Residual deformation of fragments of the embryonic tissues preserved after relaxation of the stretching force serve as a criterion of active redistribution of their cells caused by this stretching. We measured residual deformations of the Xenopus laevis ventral and dorsal ectoderm at the early gastrula and lateral ectoderm at the late gastrula-early neurula after stretching of varying time and force. While the samples responded to moderate (up to 40%) short-term stretching as elastic bodies (residual deformations were absent), residual deformation appeared in the early gastrula tissues after 30–60-min stretching, which were more pronounced in the ventral tissues than in the dorsal ones. On the contrary, a contractile reaction developed in the late gastrula-early neurula tissues in response to 60-min stretching, which almost relaxed residual deformation within 20 min after unloading. A conclusion was drawn that gastrulation and neurulation proceed under the conditions of relaxing and nonrelaxing mechanical tensions, respectively. Mechanical bases and morphogenetic role of the described reactions is discussed.     

Skin viscoelasticity studied in vitro by microprobe-based techniques

Time-dependent deformation of porcine skin was studied in vitro using specialized microprobe instruments. The deformation behavior of stratum corneum, dermis, and whole skin is examined in the context of results of creep strain, elastic stiffness, and viscoelastic constants obtained in terms of the hold time, loading/unloading rate, and maximum indentation depth (load). Skin time-dependent deformation is significantly influenced by dermis viscoelasticity up to a critical indentation depth (load) beyond which it is controlled by the outermost hard epidermis, particularly stratum corneum. Skin viscoelastic behavior under constant load (creep) and constant displacement (stress relaxation) is interpreted in the light of phenomenological observations and experimental trends.     

An inelastic multi-mechanism constitutive equation for cerebral arterial tissue

Intracranial aneurysms (ICA) are abnormal saccular dilations of cerebral arteries, commonly found at apices of arterial bifurcations and outer walls of curved arterial segments. Histological evidence suggests the stages in ICA development include the deformation of a segment of arterial wall into a “bleb” with no identifiable neck region followed by the development of an aneurysm with a clear neck. Afterwards, the aneurysm may undergo further enlargement, possibly with significant biological response including calcification and thrombosis. Past studies of the biomechanics of cerebral aneurysm tissue have been directed at modeling elastic deformations of pre-existing aneurysms. Taking this approach, the aneurysm wall is treated as a different entity than the arterial tissue from which it developed. In the current work, a nonlinear, inelastic, dual-mechanism constitutive equation for cerebral arterial tissue is developed. It is the first to model the recruitment of collagen fibers and degradation of the internal elastic lamina, two important characteristics of early stage aneurysm formation.     

Fibulin-4: a novel gene for an autosomal recessive cutis laxa syndrome

Cutis laxa is a condition characterized by redundant, pendulous, and inelastic skin. We identified a patient with recessive inheritance of a missense mutation (169G-->A; E57K) in the Fibulin-4 gene. She had multiple bone fractures at birth and was diagnosed with cutis laxa, vascular tortuosity, ascending aortic aneurysm, developmental emphysema, inguinal and diaphragmatic hernia, joint laxity, and pectus excavatum by age 2 years. Her skin showed markedly underdeveloped elastic fibers, and the extracellular matrix laid down by her skin fibroblasts contained dramatically reduced amounts of fibulin-4. We conclude that fibulin-4 is necessary for elastic fiber formation and connective tissue development.     

Structural dynamics and resonance in plants with nonlinear stiffness

Although most biomaterials are characterized by strong stiffness nonlinearities, the majority of studies of plant biomechanics and structural dynamics focus on the linear elastic range of their behavior. In this paper, the effects of hardening (elastic modulus increases with strain) and softening (elastic modulus decreases with strain) nonlinearities on the structural dynamics of plant stems are investigated. A number of recent studies suggest that trees, crops, and other plants often uproot or snap when they are forced by gusting winds or waves at their natural frequency. This can be attributed to the fact that the deflections of the plant, and hence mechanical stresses along the stem and root system, are greatest during resonance. To better understand the effect of nonlinear stiffness on the resonant behavior of plants, plant stems have been modeled here as forced Duffing oscillators with softening or hardening nonlinearities. The results of this study suggest that the resonant behavior of plants with nonlinear stiffness is substantially different from that predicted by linear models of plant structural dynamics. Parameter values were considered over a range relevant to most plants. The maximum amplitudes of deflection of the plant stem were calculated numerically for forcing frequencies ranging from zero to twice the natural frequency. For hardening nonlinearities, the resonant behavior was 'pushed' to higher frequencies, and the maximum deflection amplitudes were lower than for the linear case. For softening nonlinearities, the resonant behavior was pushed to lower frequencies, and the maximum deflection amplitudes were higher than for the linear case. These nonlinearities could be beneficial or detrimental to the stability of the plant, depending on the environment. Damping had the effect of drastically decreasing deflection amplitudes and reducing the effect of the nonlinearities.     

A quantitative study of regional and stage specific reaction of African clawed frog embryonic tissues on mechanical stress

Residual deformation of fragments of the embryonic tissues preserved after relaxation of the stretching force serve as a criterion of active redistribution of their cells caused by this stretching. We measured residual deformations of the Xenopus laevis ventral and dorsal ectoderm at the early gastrula and lateral ectoderm at the late gastrula-early neurula after stretching of varying time and force. While the samples responded to moderate (up to 40%) short-term stretching as elastic bodies (residual deformations were absent), residual deformation appeared in the early gastrula tissues after 30-60-min stretching, which were more pronounced in the ventral tissues than in the dorsal ones. On the contrary, a contractile reaction developed in the late gastrula-early neurula tissues in response to 60-min stretching, which almost relaxed residual deformation within 20 min after unloading. A conclusion was drawn that gastrulation and neurulation proceed under the conditions of relaxing and nonrelaxing mechanical tensions, respectively. Mechanical bases and morphogenetic role of the described reactions is discussed.     

Structural changes of skin and amnion grafts for transplantation purposes following different doses of irradiation

An important part of the preparation of biological material for transplantation is sterilization. The aim of our study was to assess the impact of ionizing radiation on three types of biological tissues and the impact of different doses on cells and extracellular matrix. Three types of frozen tissues (porcine skin xenografts, human skin allografts and human amnion) were divided into five groups, control and groups according to the dose of radiation to which these samples were exposed (12.5, 25, 35 and 50 kGy). The tissue samples were fixed by formalin, processed by routine paraffin technique and stained with hematoxylin and eosin, alcian blue at pH 2.5, orcein, periodic acid schiff reaction and silver impregnation. The staining with hematoxylin and eosin showed hydropic degeneration of the cells of epidermis in xenografts by the dose of 12.5 kGy, in human skin it was observed by the dose of 35 kGy. The staining for elastic fibers revealed damage of fine elastic fibers in the xenografts dermis by the dose of 12.5 kGy, in the allografts by 35 kGy. Another change was the disintegration of basement membrane of epithelium, especially in the human amnion at the dose of 50 kGy. The silver impregnation visualized nuclear chromatin condensation mainly in human amnion at the dose of 12.5 kGy. Our results have shown that the porcine xenografts and human amnion were more sensitive to irradiation than the human skin. In the next phase of the project we will focus at more detailed changes in the tissues using immunohistochemical techniques.