In performing spinal fusion, there are many factors that might cause iatrogenic and degenerative changes to the back muscle. In particular, conventional open surgery can cause ischemia and denervation of the back muscle due to the need to make a long skin incision and also from the long hours of excessive pressure during the process of retracting the back muscle from the spinous process1,2
. According to Kawaguchi et al.3-5
, back muscle injury at the time of posterior lumbar surgery is related to the operation time and retraction pressure. Therefore, it is recommended that the retraction be released for 5 minutes after 1 hour of retraction to prevent serious back muscle injury. Gejo et al.14
also reported that the muscle retraction time influences the osteoperative back muscle function, and more than 2 hours pressure after retraction is believed to reduce the flow of capillary vessels, causing ischemic intramuscular changes15-17
. Recently, as a solution for these issues, MI-TLIF has attracted considerable attention because it minimizes the level of soft tissue injury during surgery, surrounding muscle injury and blood loss during surgery6,7
. Stevens et al.18
compared the pressure applied to the muscles between the two approaches using a cadaver with a minimally invasive approach and reported a lower value with MI-TLIF.
In this study, a relatively long skin incision was applied to detach the back muscle from the bones, which is known as conventional open surgery, and considerable pressure was applied for many hours when all surgical methods were performed to retract both sides of the back muscle, which may cause injury or degeneration. On the other hand, a relatively short skin incision was made in MI-TLIF with a blunt dissection of the space between the muscles, which reduces the direct muscle injury, and the time for back muscle retraction is relatively short and there is a lower degree of pressure because the surgery is performed through 2 incisions on the site. Therefore, methods to quantify them have been examined with the aim of reducing the level of back muscle degeneration.
Hyun et al.19
reported that the paramedian interfascial approach may preserve the back muscle even more by comparing the degree of back muscle injury according to the midline and paramedian approaches in lumbar fusion through postoperative quantification of back muscle mass using Computed tomography in 26 patients. However, computed tomography has difficulty in observing fat infiltrations, degenerative changes, and internal structures in patients who have undergone fixation by inserting metallic objects due to the artifacts20
. Therefore, many attempts have been made to evaluate the degree of muscle injury by measuring the degree of back muscle injury with a serological evaluation after diverse lumbar surgeries1,21
. Kim et al.8
reported that in lumbar fusion, the minimally invasive approach contributes considerably to reducing the level of postoperative muscular injury and systemic inflammatory response compared to the temporary approach through quantitative analysis that employs serum enzymes, such as creatine phosphokinase, inflammatory cytokine etc. After operating on 17 cases of MI-TLIF and 18 cases of posterior lumber fusion (PLF), Starkweather et al.22
reported that after a 6-week follow-up visit, the Interleukin-6 levels were also significantly higher in the MI-TLIF group than in those patients who had undergone PLF, which may be an indicator of ongoing nerve regeneration and healing. Serological analysis may be used to evaluate an immediate postoperative muscular injury21
. However, as time passes after the injury, metabolic equilibrium is formed where the figures are normalized making a long term observation difficult. Another study measured the degree of back muscle degeneration using MRI12,23,24
. MRI is not only capable of measuring the muscle mass9
but can also calculate the fat content of the skeletal muscles10,11
thus enabling measurements of the degree of fat infiltration before and after surgery. After surgery using the traditional and minimally invasive approaches in 4 cases each, and carrying out a follow up evaluation of the extent of damage and edema in the multifidus muscle using MRI, Stevens et al.18
reported a remarkable decrease in edema of the multifidus muscle but no significant difference in muscle contraction.
In this experiment study, in 48 cases in whom conventional open surgery and MI-TLIF had been performed, a quantitative analysis of the degenerative changes in the back muscle after 1 year revealed that MI-TLIF is effective in diminishing the changes in the percentile value of fat infiltration of the back muscle and the proportion of crosssectional area compared to the conventional open surgery. In particular, in a group where a multi-segment fusion had been performed, the difference in the surgical approach was clear as compared to the group where a single segment fusion had been performed. However, the difference was not significant. In addition to the direct intraoperative injury to the back muscle, factors, such as the postoperative bed rest, period of brace use and degree of activity, should also affect the degenerative changes in the back muscle25,26
. In addition, from a long-term perspective, postoperative management might play a role as the key parameter for the degeneration of the back muscle.
A radiologic evaluation has limitations in accurately reflecting the actual clinical status. Therefore, further study will be needed to analyze the clinical assessment specifications and muscle activity. In this study, it could not be demonstrated whether the degree of back muscle degeneration reached statistical significance in both groups. Follow-up studies are warranted because of the possibility that this might have originated from a type II error (βerror)27
due to the small sample size.