Posterior Approach for Debridement of the Psoas Abscess

Summary

This is a cadaveric study investigating the landmarks for the posterior approach for irrigation and debridement of the psoas abscess. The interval between the transverse processes (TP) was used to access the substance of the psoas muscle.

Abstract

This method focuses on outlining a safe zone for irrigation and debridement of a psoas abscess through a posterior approach. Initially, an anterior approach to the spine was performed to ensure that the anterior longitudinal ligament and the psoas muscle could be visualized. All the abdominal organs were removed. Subsequently, a posterior approach was performed to remove the paraspinal muscles from L1–L5. The transverse processes, pars interarticularis and lamina of L1–L5 were identified. The exiting nerve root was identified between the transverse processes and followed into the substance of the psoas muscle. Using the anterior and posterior approach, the lumbar plexus was isolated from the substance of the psoas muscle. Before and after various steps of dissection, digital photographs were obtained. These images were uploaded into ImageJ and multiple measurements, including the distance between the lateral superior and inferior tip of each TP to the most lateral region of the plexus, the distance between the lateral superior and inferior tip of the TP to the lateral edge of the psoas, and the width of the lumbar plexus were recorded. The safe zone for entering the substance of the psoas muscle was defined between the lateral edge of the psoas muscle and the lateral edge of the lumbar plexus. The relationship of this interval to the tip of the transverse process at each level was measured and reported.

Introduction

A psoas abscess (PA) is a rare diagnosis reported in 0.4–1 per 10,000 admissions to the hospital^1,2. A PA can be caused by hematogenous spread, lymphatic spread from a distant infectious site, or by contiguous spread from adjacent organs³. Currently, percutaneous drainage under real-time computed tomography is a safe and first-line treatment for a PA⁴. Dietrich et al. found that it has a higher success rate compared to open surgery, offering a lower mortality rate and shorter hospital length of stay². However, Tabrizian et al. found that 44% of the patients treated with percutaneous drainage underwent open surgery to eradicate the infection⁵.

Given the rate of open surgeries performed for treatment of psoas abscesses, the aim of the method developed in this study is to find a "safe zone" for irrigation and debridement (I&D) of a psoas abscess via a dorsal approach. An initial ventral approach is taken to identify and isolate key anatomical structures, including the transverse processes (TP), psoas muscle, and lumbar plexus (LP). A dorsal approach is then taken to further isolate the TP and LP from the surrounding structures. Throughout the various steps of dissection, photographs are obtained. Once the dissection is complete, the images are uploaded to ImageJ (1.48v, National Institute of Health) and several measurements are taken to identify the relationship between the TP, LP, and psoas muscle. Finally, using the measurements obtained, a "safe zone" for entering the substance of the psoas and minimizing the risk of lumbar plexus injury is calculated. To our knowledge, this is the first work that uses the transverse processes as reference points for the purpose of finding a safe zone for I&D of a psoas abscess.

Protocol

The use of deceased human specimens, for the purposes of developing this method, has been approved by the WellStar Research Institute's institutional review board and conforms with the ethical guidelines established by the institution's ethics committee. The specimens used in this method were all formalin-fixed, transected at the T12 vertebral body, and had their abdominal organs and viscera removed.

1. Preparation of the cadaveric specimen and surgical field

1. Place the previously thawed cadaveric specimen on the surgical table in the supine position and illuminate the field with surgical lights. Cover the specimen with a blanket, sparing the upper and lower abdomen.

2. Isolation and identification of key anatomical structures using a ventral approach

1. Beginning with a ventral approach, use a blade to make a midline incision, cranial to caudal, down to the hypogastric region and directly perpendicular to the anterior-superior iliac spines. Make a transverse cut between the ASIS and proceed to use a surgical blade and scissors to reflect and remove the abdominal musculature and fascia. This will improve visualization and aid in measurement accuracy.
2. Identify the transverse processes, lamina, pars interarticularis of lumbar vertebrae L1–L5, psoas muscles, and the nerve roots of the LP exiting the neuroforamina. Ensure that they are intact and undamaged (Figure 1). If any obvious damage to the hemivertebrae or transverse process is found, do not use the specimen.
3. Once all key anatomic structures are identified, carefully turn over the specimen to begin the dorsal approach.

3. Further isolation of the transverse processes and obtaining digital photographs for analysis

1. With the specimen now in the prone position, use a surgical blade, scissors, cobb elevator, and other surgical tools to resect the latissimus dorsi and paraspinal muscles, starting sequentially from the posterior spinous process until reaching the transverse processes.
2. Use the cobb elevator and other surgical tools to isolate the TPs from any remaining soft tissue attachments to aid in visualization and accurate measurements.
3. With the paraspinal muscles now removed, turn the specimen back to the supine position. Place a surgical ruler parallel to the surgical field and take digital photographs of the intact psoas muscle to demonstrate its relationship to the TP at each vertebral level.
4. Once all photographs are taken and deemed of good quality, use a surgical blade to strip the psoas muscle from its medial attachment at each lumbar body and perform a blunt dissection using blunt edged scissors or hemostats to carefully expose the entire lumbar plexus and reflect the psoas muscle laterally.
5. With the surgical ruler placed parallel to the surgical field, take additional photographs to capture the relationship of the lumbar plexus to the transverse process at each level.
6. Finally, turn the specimen on its side and take additional photographs to demonstrate the relationship of the TP to the LP in the sagittal plane.

4. Calibrating and taking measurements using ImageJ software

1. In the File tab in ImageJ (1.48v, National Institute of Health), select Open and, in the new window that appears, select the photograph to be analyzed.
2. With the desired image now open, use the "magnifying glass" tool to zoom in on the surgical ruler in the photograph. Use the line drawing tool to carefully draw a straight line between the margins of a 1-millimeter segment on the ruler.
3. Within the Analyze tab in ImageJ, select Set scale…. In the Set Scale window, set the known distance to 1.00 and the unit of length to mm. Place a check in the box next to Global and click OK. Your measurements will now be calibrated according to the number of pixels per millimeter on the ruler.

5. Calculating the "safe zone" for maximal I&D

1. Using the "line drawing" tool in ImageJ, draw a straight line between the lateral superior tip of each TP to the lateral edge of the psoas at the same level.
2. Once the lines are drawn, under the Analyze tab, select measure. A new Results window will open.
3. The measurements, in millimeters, will appear in the new window under the Length column in the order taken. Record these numbers.
4. Repeat steps 5.1–5.3 to obtain the following measurements:
   -Lateral inferior tip of each TP to the lateral edge of the psoas muscle
   -Lateral superior tip of each TP to the most lateral region of the lumbar plexus at the same level
   -Lateral inferior tip of each TP to the most lateral region of the lumbar plexus at the same level
   -Width of the lumbar plexus
   -Depth of the lumbar plexus from the superior tip of each TP to the lumbar plexus at the same level using the lateral photographs
   -Depth of the lumbar plexus from the inferior tip of each TP at the same level using the lateral photographs

6. Calculating the safe zone for maximal irrigation and debridement using the measurements obtained in Section 5

1. The safe zone for lateral to medial debridement is defined by the distance from the lateral edge of the lumbar plexus to the lateral edge of the psoas at the same level. To calculate this distance, subtract the distance from the superior and inferior tips of the TP to the lumbar plexus from the distance of the superior and inferior tips of the TP to the lateral edge of the psoas muscle. These values will correspond to the safe zone for lateral to medial debridement, in the coronal plane, at each corresponding level.
2. Define the zone of maximal irrigation and debridement in the sagittal plane as the depth of the lumbar plexus, in relation to the TP, at the corresponding level.

Representative Results

Eleven formalin-fixed cadavers were included, with an average age of 80.5 years⁶. Two cadavers had damaged lumbar vertebrae on the right side and were removed. Thus, a total of 20 measurements were included, 13 female and 7 male. The medial border of the safe zone, defined by the most lateral aspect of the lumbar plexus, was found to be approximately 1 cm medial to the tip of the TP at L1–L3, but narrowed down to approximately 6 and 1 mm at L4 and L5, respectively. The medial border of the safe zone was then subtracted by the distances of the cranial and caudal tip of each TP to the lateral edge of the psoas muscle at each level. This calculation gave us the medial to lateral area of safe debridement. The average area of the safe zone for debridement, at all levels of the lumbar spine, was found to be between 4 and 8 mm.

The area for maximum medial to lateral irrigation and debridement in the coronal plane was determined by measuring the distance from the superior and inferior tips of the TP to the lateral edge of the psoas muscle at the corresponding vertebral level. On average, the maximal extent of lateral debridement one can perform is as follows: 5.5 and 5.7 mm medial to the inferior and superior tips of the TP at L1, 4.7 and 5.1 mm medial at L2, 1.8 and 2.5 mm medial at L3, 0 and 0.4 mm medial at L4, and 3.8 and 3.7 mm lateral at L5. In the sagittal plane, the lumbar plexus was approximately 15–20 mm anterior to the TP. (Table 1).

Figure 1: Cadaveric image of an anterior approach dissection. The TP, LP and psoas are clearly defined and used as the reference points for measurements. TP = transverse process; LP = lumbar plexus; Ps = lateral edge of psoas; L3, L4, L5 = lumbar vertebrae. Please click here to view a larger version of this figure.

		Lumbar Vertebra
Measurement		L1	L2	L3	L4	L5
Lateral superior tip of TP to lateral edge of psoas		-5.7 ± 1.92	-5.11 ± 1.84	-2.52 ± 2.63	-0.42 ± 2.39	3.70 ± 1.60
Lateral inferior tip of TP to lateral edge of psoas		-5.49 ± 2.02	-4.70 ± 1.71	-1.84 ± 2.56	-0.03 ± 2.31	3.77 ± 1.57
Lateral superior tip of TP to lateral edge of LP		-9.3 ± 1.94	-11.75 ± 1.59	-10.47 ± 2.90	-6.64 ± 2.53	-1.01 ± 2.31
Lateral inperior tip of TP to lateral edge of LP		-9.2 ± 2.06	-11.71 ± 1.54	-9.8 ± 2.47	6.24 ± 2.43	-0.89 ± 2.40
Lateral edge of LP to lateral edge of psoas calculated using the lateral superior tip of the TP		3.96 ± 0.98	6.64 ± 1.17	7.94 ± 2.00	6.22 ±1.28	4.71 ± 1.38
Lateral edge of LP to lateral edge of psoas calculated using the lateral inferior tip of the TP		4.03 ± 1.05	7.0 ± 1.15	7.96 ± 2.00	6.21 ± 1.77	4.66 ± 1.59
Lateral superior tip of the TP to the LP in the saggital plane		17.89 ± 2.72	19.08 ± 1.93	19.50 ± 3.12	16.26 ± 2.69	14.97 ± 1.90
Lateral inferior tip of the TP to the LP in the saggital plane		17.84 ± 3.03	18.65 ± 1.60	19.19 ± 3.06	16.27 ± 2.93	14.92 ± 1.85

Table 1: Mean value in millimeters of 20 sets of measurements, 13 female and 7 male. Negative values indicate that the measurement is medial to the tip of the TP. The distance of the lateral edge of the psoas to the most lateral region of the LP defines the safe zone for medial to lateral debridement. The distance from the tip of the TP to the LP in the sagittal plane defines the safe zone for debridement in the sagittal plane.

Discussion

The most critical steps for finding the safe zone for irrigation and debridement of a psoas muscle abscess through a dorsal approach are 1) careful blunt dissection of the lumbar plexus during the ventral and dorsal approach; 2) preservation of the transverse processes during their isolation, as they can be fragile, for better visualization; and 3) calibrating the ImageJ software with the surgical ruler and taking careful measurements to elucidate the relationship between the TP, LP, and psoas muscle.

Surgical instruments used for dissection can be changed according to personal preference; it is crucial however to perform a careful and thorough dissection of the lumbar plexus and to ensure no damage is done to the transverse processes. When taking the photographs, make sure that the ruler is parallel to the field of dissection and that the pictures taken are of high quality. Use of both the cranial and caudal tip of the TP should be included for precise measurements.

A significant limitation of this study is a high probability of psoas muscle atrophy in the cadaveric specimens used. The average age of the specimens used was 80.5 years and cross-sectional area of skeletal muscle has been shown to be negatively correlated with age⁷. In addition, the cadavers used were formalin-fixed, leading to less tissue pliability as compared to the natural tissue that would be found in a living human being needing an operation. Another limitation is that, because their course can be highly variable after exiting the neuroforamina, the sensory nerves were not included when measuring the lumbar plexus⁸. Care should still be taken to avoid these nerves when performing I&D of a psoas abscess.

This method focuses on different measurements than previous studies, which may account for the homogenous distance found from the tip of each TP to the lateral edge of the psoas muscle. Previous methods studying psoas anatomy, such as in the studies by Spivak et al., Reid et al., Hanson et al., and Ilayperuma et al., directly measure the width of the psoas muscle^9,10,11,12. Compared to the study by Spivak et al., for example, the range of measurements found by measuring from the TP to the psoas using this method was less than the range found when measuring the psoas muscle directly. An increase of 5 mm from L1–L4 and 8 mm from L1–L5 was found when measuring from the TP to the psoas; with a direct measurement, Spivak et. al. found an increase from 15.9 mm at L2–L3 to 23.3 mm at L3–L4 and 28.7 mm at L4–L5⁹. Analysis of the data obtained indicates that the psoas muscle is generally medial to the TP and that aiming laterally would not allow access unless you are at the level of L5. Irrigation and debridement less than 1.5 cm past the transverse process in the sagittal plane should allow for avoidance of the lumbar plexus. However, pathologically, a psoas abscess may expand beyond the normal anatomical margins and a more lateral approach may be possible. MRI is recommended to clearly define the depth and medial to lateral extent of the psoas abscess and a future radiological study would be beneficial to further assess the findings of this method.

Using the measurements found in this study as a guide, one can determine the ability to safely perform irrigation and debridement of a psoas abscess, through a dorsal spinal approach, by staying within the confines of the safe zone at each level and reduce the risk lumbar plexus injury.

Materials

Name	Company	Catalog Number	Comments
Blunt ended dissecting scissors			To cover the specimens
Camera	Sony a7 III	ILCE-7
Cobb elevator	Sklar	40-6950	For isolation of TP and general dissection
Dissection table
Formalin fixed cadavers	Restore Life USA	N/A	Transected at T12 with abdominal organs removed
Hemostats
Retractors			For blunt dissection
Rongeur	Sklar	40-4085
Scissors	Sklar	15-2555
Surgical absorbent pads			Placed under the cadaver to absorb fluids
Surgical blades/scalpels	Dynarex	4110
Surgical drape/blanket			For blunt dissection
Surgical gauze sponges
Surgical lights
Surgical Ruler	Aspen Surgical Products	42182702
Tissue forceps
Tool tray