The impact of the stretching force of incision on early clinical results after primary total knee arthroplasty (TKA)

BackgroundA variety of surgical approaches are used in primary total knee arthroplasty (TKA), including medial parapatellar, sub-vastus, mid-vastus, lateral, and less invasive techniques. Previous studies demonstrated that there existed widely varied degrees of anterior knee soft-tissue stretching in patients undergoing primary TKA. However, the impact of incision stretching on TKA’s results failed to be investigated.Methods1210 patients accepted primary and unilateral TKA at the authors’ affiliated institutions between Jan. 2011 and Dec. 2015. We recorded and analyzed the following data, including each patient's characteristics, incision stretching index (IS index), perioperative information, and follow-up assessments. By trisecting the IS index, patients were grouped and compared about visual analogue scale (VAS) pain scores as the primary outcome, knee circumference, knee range of motion (ROM), sensory testing, and the strength of quadriceps. Results1089 patients undergoing primary and unilateral TKA in our two institutions were screened for final analysis, and 121 ones were excluded. The mean IS index was 22.6 (range: 19.7-24.8). VAS pain score, knee circumference, ROM, area of abnormal sensation, and the strength of quadriceps among group IS A, IS B, and IS C were significantly different (P<0.05). With the increase in the IS index, VAS pain score, knee circumference, area of abnormal sensation, and incision problems were significantly increased (P<0.05), while ROM and the strength of quadriceps decreased (P<0.05). Also, no significant difference in PJI and deep vein thrombosis (DVT) among groups was observed (P>0.05). Conclusions There was a significant impact of incision stretching on clinical outcomes after TKA. Decreased incision stretching, which mainly depended on the length in full extension of the knee, can improve postoperative pain relief, surgical swelling, ROM, sensory disturbance of the knee, and the strength of quadriceps with reduced risk of incision complications. length in flexion to extension in patients undergoing primary TKA. Furthermore, we included the IS index as an assessment criterion to determine how the difference between EL and FL affected TKA’s outcomes. We compared VAS pain score, ROM, QS, knee circumference, and incision problems among groups. The present study revealed that pain feeling, ROM, QS, swelling, and incision problems were improved better two weeks after TKA with a decrease in IS index. These results showed that the closer the gap between EL and FL was, the less the incision stretching was,which benefit outcomes after TKA. Our present results suggested IS A was the optimal better incision healing, and hence it is clinically important. Our results may provide clinical evidence and basic data of the obvious effects of incision stretching on key outcomes after TKA, such as pain, ROM, QS, and swelling. Furthermore, our results reveal a greater surface area of sensory change in


Introduction
Total knee arthroplasty (TKA) is regarded as a successful surgical method to treat the severe degenerate tibiofemoral joint. While total knee arthroplasty has long been judged primarily by implant survivorship, the focus is increasingly on patient-reported outcomes [1]. Reconstructive surgeons use several methods to improve patient outcomes following total knee arthroplasty, which include surgical closure [2,3], cocktail therapy [4], use of tranexamic acid [5,6], and cryotherapy [7]. Those are effective treatments to achieve improved outcomes according to previous studies.
A variety of surgical approaches are used in TKA, including medial parapatellar, sub-vastus, midvastus, and lateral techniques [8][9][10][11]. These approaches are typically applied through incisions up to 20-30 cm in length and offer optimal exposure of the entire knee joint [12]. Minimally invasive approaches for TKA have become popular and often require specialized training, modified retractors, and modular implants. In all approaches, a certain amount of skin stretching occurs to expose the knee adequately [13,14]. Nikolaos demonstrated varying degrees of anterior soft-tissue stretching during primary TKA [14], but the impact of this variation in tension has yet to be investigated and is not fully understood.
We assumed that the length of skin incision determines incision stretching, which would affect recovery from TKA. To our best knowledge, no study has examined that point. Therefore, the present study was conducted to investigate the optimal incision length, that is, appropriate incision stretching in patients undergoing primary TKA.

Materials And Methods
Patients and Data 1210 patients accepted primary and unilateral TKA at the authors' affiliated institutions between Jan. 2011 and Dec. 2015. This study had been approved by the ethics committee of the authors' affiliated institutions and accorded with the declaration of Helsinki. Patients who accepted primary and unilateral TKA due to OA were included. Excluded cases including the patients who were unable to complete necessary follow-up; have a knee with preoperative flexion contracture greater than 15°a nd preoperative flexion less than 90°; have paresthesia in lower limb; have any scar within the knee area; have patella alta or baja; dead after surgery. We recorded and analyzed the following data, including each patient's characteristics, perioperative information, and follow-up assessments. These data were completed and evaluated by respective experienced therapists who were blinded to this research-based on uniform standards.
We calculated the incision stretching index (IS index), which is defined as FL-EL/EL in the present study. FL represented the length of surgical incision when the knee was flexed at 120°. EL represented the length of surgical incision when the knee was extended at 0°. The length of surgical incision was measured intraoperatively using a sterile tape with care taken to avoid crinkling. The primary outcome measure was visual analogue scale (VAS) pain score rated on a scale of 0-10 from no pain to severe pain. Secondary outcome measures include knee girth reflecting postoperative swelling, knee range of motion (ROM), sensory testing, and the strength of quadriceps. Knee circumference was measured on the proximal pole of the patella using a tape. ROM was measured using a standard hand-held goniometer with a patient in a supine position. The goniometer was placed over joint space with one arm aligned with fibular head and lateral malleolus and the other in line with the greater trochanter. The knee was flexed maximally at the patient's best, and then the angle was measured in degree. In the sensory testing, all patients were examined for the area of altered sensations to 'light touch' using pin-prick over the front of the knee based on Appendix Table 1. Table 1 Clinical parameters in each group.  After mapping the margins of the area of abnormal sensation, surgeon assistant measures that using the grid made of a thin sheet of plastic. The isokinetic strength of quadriceps (QS) was assessed using a ConTrex System Dynamometer (ConTrex MJ; CMV AG, Dübendorf, Switzerland) pre-operatively and two weeks postoperatively at 60°/s. When being evaluated, the patient exerted maximum effort for two attempts, and then the peak torque values of quadriceps were obtained. All the assessments mentioned above were completed two weeks postoperatively. Participants were required to indicate abnormal sensation from anesthesia to hypersensitive (see appendix Table 1), and we measure its area using a purpose-designed grid six months postoperatively [1]. Each sensory test was performed twice, and the mean value was finally analyzed. Significant complaints and complications over the study period would be reported to respectively experienced orthopedists and then determined whether to be analyzed finally. Incision problems include delayed union, nonunion, and skin necrosis.
Doppler ultrasonography was arranged to confirm the presence or absence of deep vein thrombosis (DVT) after surgery and before discharge. All participants who performed the above-mentioned assignments were blinded to the final analysis.

Surgical Procedure
All operations were performed by one surgical team in respective institutions using epidural anesthesia alone, general anesthesia alone, regional anesthesia alone, or combined general and    The effect of IS index on clinical outcomes after primary TKA VAS pain score, knee circumference, ROM, area of abnormal sensation, and the strength of quadriceps among group IS A, IS B, and IS C were significantly different (P < 0.05). With the increase in the IS index, VAS pain score, knee girth and the area of abnormal sensation were significantly increased (P < 0.05), while ROM and the strength of quadriceps decreased (P < 0.05) ( Table 2).

Complications
With the increase in the IS index, the number of patients with incision problems was increased significantly (P < 0.05) ( Table 2). Besides, no significant difference in PJI and DVT among groups was observed (P > 0.05) ( Table 2).

Discussion
In this study, we investigated the outcome related to the amount of stretch placed on the skin incision as measured by the ratio of the incision length in flexion to extension in patients undergoing primary TKA. Furthermore, we included the IS index as an assessment criterion to determine how the difference between EL and FL affected TKA's outcomes. We compared VAS pain score, ROM, QS, knee circumference, and incision problems among groups. The present study revealed that pain feeling, ROM, QS, swelling, and incision problems were improved better two weeks after TKA with a decrease in IS index. These results showed that the closer the gap between EL and FL was, the less the incision stretching was,which benefit outcomes after TKA. Our present results suggested IS A was the optimal ratio between flexion and extension length in patients undergoing primary TKA. Given the above, during the study, we observed that such an incision (IS A) that the quadriceps tendon was incised 4 cm proximally above the upper border of the patella and distally along the medial side of the patellar tendon to the tubercle of the tibia could improve the results after TKA.
There are primarily three reasons that explain the above findings. Firstly, the strength of quadriceps is decreased by up to 60% after TKA, as the cutting of quadriceps muscle, eversion of the patella, and extreme knee flexion during TKA [18]. In addition to that, K. Chareancholvanich et al. reported that there is a significant effect of the length of surgical incision on the postoperative strength of quadriceps [18]. What's more, the size of incision directly affects pain feeling and swelling as larger incision produces more inflammatory factors [16]. Less pain decreased swelling, and better strength of quadriceps makes patients relatively comfortable while they perform the functional exercise.
Finally, IS C represents the ratio at the highest stretch level in comparison to IS A and B, which make surrounding tissue tolerate the largest tensile force, especially as the knee is in an extreme flexion intraoperatively. Made up of a complex network of collagen and elastin fibers, the skin has elastic characteristics and mechanical strength within its physiological limits [17]. However, previous studies indicated that the acute, purely reversible elastic response of stretched skin tissue, which is similar to the stretching process of the incision during TKA would be impaired to a certain extent if we stretched it beyond its physiological limits [17]. This impairment by releasing inflammatory elements and strain injury of nerves showed pain feelings, paresthesia, and tissue edema. Therefore, the contents above can help explain why IS A is the optimal ratio of the incision length in flexion to extension in patients undergoing primary TKA.
Clinically, more incision problems, even including skin necrosis, occurred as the incision was at higher stretch levels. Therefore, to avoid these problems, a decrease in incision stretching force by increasing its length in extension, which improves microcirculation of surrounding tissue, leads to a better incision healing, and hence it is clinically important. Our results may provide clinical evidence and basic data of the obvious effects of incision stretching on key outcomes after TKA, such as pain, ROM, QS, and swelling. Furthermore, our results reveal a greater surface area of sensory change in the front of the knee following TKA occurs in patients with incision at higher stretch level, since nerves around the knee which were stretched beyond physiological limits were shown to be impaired more widely due to the characteristic of the extensibility of the axon [1]. This greater alteration in skin sensation discounted the ability to kneel due to the fear of harming the prosthesis [1], combined a negative effect on subjective feelings such as titillation, results in patient dissatisfaction following TKA. That is, this study suggests that the length of incision, in extension, is possibly an effective treatment method, especially when patients perform the painful exercise, as appropriate incision stretching makes them comfortable by less pain and improved strength of quadriceps. It is conceivable and desirable that there were medical costs saved due to earlier recovery. Therefore, the findings in the present study suggest that selecting the optimal ratio of the incision length in flexion to extension in extension, which determines incision stretching would be significant in the clinical practice.
To our best knowledge, although there merely two studies involving the incision length in primary TKA. However, their results have a certain limitation. K. Chareancholvanich's report only investigated the effect of the length of knee incision in full extension on quadriceps strength [16]. Despite elaborate data regarding quadriceps strength following TKA in his report, only different incision length in extension was discussed. However, incision length in flexion, namely the problem of incision stretching, did not be studied. By contrast, Roidis NT's study paid close attention to the incision stretching in patients undergoing primary TKA [14]. He reported that the incision length was 5.7 cm longer in flexion than extension. The surgical incision site stretched an average of 23.6% in flexion compared to in extension [14]. However, what impact could incision stretching have on clinical outcomes after TKA, such as pain, ROM, and swelling, has not been investigated yet. Based on these two studies, our study further deepens the significance of incision stretching, and this is the first study to investigate that. A comprehensive understanding of the impact of incision stretching on TKA's results may help surgeons to optimize clinical practice. This helps us know the effect of incision stretching on TKA's results we interested in. Findings from this study can guide surgeons to attach importance to the length of incision in extension, which determines the stretching force of your incision. Despite being the seemingly minor problem, it is a real concern we should deliberate on. It becomes a well treatment of fast recovery from TKA with the circumstance of less incision stretching.
We hope the present results could be helpful in optimizing TKA's detail and perfect clinical outcomes.
Several limitations in this study warrant discussion. First, our data of incision stretching was obtained by calculating the self-designed formula: FL-EL/EL, and thus the examination of its real size was indirectly and relatively inaccurate. Second, no pathological examination was applied. However, that serves as the golden criterion, which demonstrates the incision stretching exists by the observation of changes in the microscopic structure, such as the disruption of skin collagen bundles. Third, our study employs many different combinations, and this made pain score evaluation accurate to a certain extent. Finally, relatively small sample results in less persuasive conclusions. Therefore, in the future study, we intend to quantitatively evaluate changes in the tissue under different stretching forces via microscopic examination and objective measurements such as a mechanical sensor.

Conclusion
In the present study, there was a significant impact of incision stretching on clinical results after TKA.
Appropriate incision stretching mainly depended on the length in full extension of the knee, which can benefit on postoperative pain, surgical swelling, ROM, abnormal sensation, the strength of quadriceps and patient's perception of recovery from TKA with the decreased risks of incision complications.

Consent for publication
TM and GL participated in concept development, data generation, quality control of the data, data analysis and interpretation, and writing of the manuscript. TM, SY, and YP were responsible for the data analysis and participated in the interpretation and presentation of the data. SJ and GL provided input into the data interpretation. TM, GL, and YP were involved in the concept development, quality control of the data, and data analysis and interpretation of the manuscript. All authors have read and approved the final version of the submitted manuscript.