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Neurovascular risk of bicortical tibial drilling for screw and spiked washer fixation of soft-tissue anterior cruciate ligament graft
Neurovascular Risk of Bicortical Tibial Drilling for Screw and Spiked Washer Fixation of Soft-Tissue Anterior Cruciate Ligament Graft William R. Post, M.D., and Stephen S. King, B.A.
Purpose: As the use of soft-tissue anterior cruciate ligament (ACL) grafts, including hamstring grafts, has become more prominent and the benefits of aggressive rehabilitation have become clear, maximizing fixation with screw and spiked washers is important. Bicortical fixation may be superior. We were concerned about potential neurovascular risks and designed this study to define the posterior neurovasculature structures at risk when drilling for bicortical tibial screw fixation during ACL reconstruction. Type of Study: Consecutive sample. Methods: We placed the tibial tunnel arthroscopically in 10 cadaveric knees using a standard tibial drill guide. Accurate tibial tunnel position was documented in each knee by lateral radiograph. A 4.5-mm bicortical drill hole was placed perpendicular to the tibial surface 1 cm distal to the tibial tunnel. The distances from the posterior tibial drill exit point to nearby neurovascular structures were measured with a caliper. Results: The closest structure to the exit point was the bifurcation of the popliteal artery/vein (11.4 ⫾ 0.6 mm; range, 8.4 to 14.0 mm). The next closest was the anterior tibial vein (11.7 ⫾ 1.6 mm; range, 3.5 to 22.8 mm). The closest any individual hole came to a neurovascular structure was 3.5 mm from the anterior tibial vein. Conclusions: Bicortical drilling for fixation of soft tissue grafts appears reasonably safe. The structures at greatest risk for injury are the bifurcation of the popliteal artery/vein and the anterior tibial vein. Key Words: ACL graft—Fixation—Neurovascular—Complication—Drilling—Bicortical.
T
endon to bone fixation using a screw and spiked washer construct has been successfully used in many orthopedic procedures, including anterior cruciate ligament (ACL) reconstruction. Methods of graft fixation in ACL reconstruction have received intense scrutiny to reach goals including high load to failure, resistance to cyclic loading, ease of clinical use, and avoidance of potential complications. As the use of soft tissue ACL grafts, including hamstring grafts, has become more prominent and the benefits of aggressive
rehabilitation have become clear, maximizing fixation with screw and spiked washers is an important consideration. Insertion torque has been shown to correlate with load to failure.1 Because of our clinical experience, which suggests greater torque is possible using bicortical fixation of the tibial screw and spiked washer, we designed this study to evaluate the safety of bicortical drilling for placement of the tibial screw.
METHODS
From the Department of Orthopedics, West Virginia University, Morgantown (W.R.P.); and Marshall University School of Medicine, Huntington, West Virginia (S.S.K.), U.S.A. Address correspondence and reprint requests to William R. Post, M.D., Section of Sports Medicine and Shoulder Surgery, Department of Orthopedics, West Virginia University, PO Box 9196, Morgantown, WV 26506-9196, U.S.A. E-mail: [email protected] © 2001 by the Arthroscopy Association of North America 0749-8063/01/1703-2460$35.00/0 doi:10.1053/jars.2001.21539
244
Ten fresh-frozen cadaveric knees (midtibia/midfemur) were selected for study. The ACL was resected arthroscopically. A tibial drill guide (Arthrex, Naples, Fl; model AR-1866, AR-1867) was inserted through the medial parapatellar portal referencing the posterior cruciate ligament, with the angle set at 60° and a 2.4-mm guidewire was drilled. The drill guide was removed and a 9-mm cannulated drill was used to create the tibial tunnel, exactly as would be done
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 3 (March), 2001: pp 244 –247
NEUROVASCULAR RISK OF BICORTICAL TIBIAL DRILLING during routine ACL reconstruction. Soft tissue was cleared from the tibia just distal to the tibial tunnel. One centimeter distal to the tibial tunnel, we drilled a bicortical 4.5-mm hole perpendicular to the tibial surface using a custom cylindrical drill guide to assure perpendicular placement as is our practice clinically (Fig 1). Lateral knee radiographs in full extension were made with a 9-mm stainless steel rod in the tibial tunnel and a 4.5-mm drill bit in place in the tibia to document accurate tunnel placement. We measured the tunnel position by measuring the distances from the center of the steel rod at the level of the tibial plateau to the anterior and posterior borders of the plateau. The anterior/posterior percentage was calculated to document tunnel location per the technique of Almekinders et al.2 (Fig 2). The skin and subcutaneous layers were dissected from the posterior leg, protecting the deep fascia and neurovascular structures. The gastrocnemius was bisected longitudinally and reflected medially and laterally. Starting inferiorly, the soleus was reflected lat-
245
erally, taking care to free it from the neurovascular bundle. We were careful to not disturb the position of the neurovascular structures. At this point, the 4.5-mm drill bit was reinserted anteriorly. The posterior drill exit site was identified. Using a digital caliper (Mitutoyo U.K. Ltd., Hampshire, England, Model CD-6“B) we measured the shortest distance from the edge of the drill exit hole to 4 different structures: the trunk of the bifurcation of the popliteal artery or vein (whichever lay closer), the anterior tibial vein, the anterior tibial artery, and the peroneal nerve. Neurovascular structures were carefully examined for evidence of any damage that might have occurred from drilling, then all soft tissue was stripped from the tibiae. To define the consistency of tibial drill hole placement, we measured the shortest distance between the edge of the drill exit hole and knee joint capsule, lateral tibial border, and tibial articular surface. Anteriorly, we measured the shortest distance from the edge of the tibial tunnel to the tibial articular surface and the tibial midline. We also measured from the drill entry hole to the lateral border of the tibial tuberosity and to tibial articular surface. Lastly, drill hole depth was measured. Each measurement was taken 3 times, resetting the caliper to zero before each measurement. The caliper was used such that each measurement was determined before observing the numeric display to avoid observer bias. Readings were averaged for each individual specimen. Average distances were calculated for the study group. Intraclass correlation coefficients were calculated for all measurements. RESULTS Radiographic Tibial Tunnel Position The mean distance from the anterior border of the tibial plateau to the center of the tibial tunnel was 47% (SD, 4.9) of the anterior to posterior length of the tibial plateau. Anatomic Observations
FIGURE 1. Anterolateral and cross-sectional views of 3.5 mm drill bit pathway and tibial tunnel position.
There were no apparent injuries or anomalies involving any neurovascular structures. In 9 of 10 specimens, the drill exit hole was inferior to the tibiofibular joint capsule. One drill penetrated the joint capsule. Nine drill exit points were superior to the bifurcation of the popliteal artery/vein into the anterior and posterior tibial artery/vein. The lone exception here was 1 specimen in which the exit was almost directly anterior to the anterior tibial vein (3.5 mm). This was the closest that any drill hole came to a
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FIGURE 2. Lateral radiograph indicating typical anteroposterior position of the 9-mm tibial tunnel. Measurements indicated are (A) total anterior-posterior tibial plateau length, (B) anterior tibial plateau border to the center of the proximal tibial tunnel, and (C) posterior tibial plateau border to the center of the proximal tibial tunnel.
Magen et al.3 recently studied several techniques of screw and spiked washer fixation and found higher yield loads and less slippage compared with interference fixation. All the techniques they studied specified drilling of the posterior tibial cortex for fixation. Because we believe that bicortical fixation is likely to produce greater torque and superior fixation, we desired to study the anatomic position of the drill exit site relative to nearby neurovascular structures. We are unaware of reports of neurovascular injury secondary to placement of a tibial fixation drill hole. Nonetheless, because of concern regarding the possibility of neurovascular injury with bicortical drilling for tibial ACL graft fixation, we measured the distance from the exit hole to nearby neurovascular structures. We found the closest neurovascular structures were, on average, more than 1 cm from the drill exit site. However, 1 drill hole emerged only 3.5 mm from the tibial vein (but did not injure the vein). In this 1 case, had the drill plunged deeply it would have touched and possibly injured the anterior tibial vein. In the other 9 specimens, had the drill plunged deeply from the exit hole, continuing deeper would not have placed the drill tip in contact with neurovascular structures. We found slight variation in the starting point for tibial tunnel placement but minimal variation of the articular entrance of the tibial tunnel, a value that met accepted radiographic standards. We believe this represents a realistic variation for an experienced ACL surgeon. Based on our data and clinical experience, we believe bicortical tibial fixation screws can be placed safely, but surgeons must remember to be particularly cautious when perforating the posterior
neurovascular structure. Average measurements taken from the surface of the exit hole to the structures in question and tibial drill hole depth are summarized in Table 1. The closest structure, on average, to the drill exit hole was the trunk of the bifurcation (average, 11.4 mm). Next closest was the anterior tibial vein (average, 11.7 mm) with mean drill hole depth being 34.5 mm (Fig 3). DISCUSSION Tibial screw and spiked washer fixation has been used with cancellous screws (without drilling the posterior cortex) and with bicortical fixation. Many of the commercially available screws are cancellous screws by design. Recently, fixation strength has been described to correlate with the degree of torque applied.1
FIGURE 3. Actual drill hole exit sites referencing the lateral border and the posterior articular surface of a typical tibia.
NEUROVASCULAR RISK OF BICORTICAL TIBIAL DRILLING
247
TABLE 1. Measurements to Neurovascular Structures, Bony Landmarks, and Drill Depth Structure Drill exit to: Popliteal artery/vein bifurcation Anterior tibial vein Anterior tibial artery Peroneal nerve Joint capsule Lateral tuberosity border Posterior articular surface Drill entry to: Articular surface Tibial mid-line Lateral tibial border Tunnel to: Articular surface Tibial mid-line Tibial drill hole depth
Mean (mm)
Std Error⫾
Range (mm)
Intraclass Correlation
11.4 11.7 16.3 34.1 11.5 6.7 35.1
0.6 1.6 1.7 1.6 1.18 1.14 1.57
8.4-14.0 3.5-22.8 9.8-26.1 24.6-41.5 17.5-5.5 0.0-10.9 27.4-43.2
0.745 0.939 0.957 0.918 N/A* 0.966 0.864
39.5 28.0 35.5
1.74 1.04 2.7
30.4-46.4 22.6-32.8 31.5-41.6
0.736 0.988 0.987
23.7 28.0 34.5
2.29 1.19 1.48
15.4-39.3 20.0-33.5 26.4-41.4
0.727 0.993 0.999
* Not available; only 1 measurement/specimen taken.
tibial cortex. Given the proximity of neurovascular structures posteriorly, it might be wise to design a screw with a blunt tip to decrease the possibility of iatrogenic injury. Although not addressed in the current study, it also makes sense that it might be safer to drill this tunnel with the knee flexed to relax any potential tension in the posterior neurovascular structures. Based on this anatomic study, bicortical screw and spiked washer fixation of soft tissue ACL grafts appears to be relatively safe. The structures at greatest risk would be the bifurcations of the popliteal artery and vein and also the anterior tibial vein. Variations in anatomy and surgical technique are possible and we recommend care in drilling through the posterior cortex.
Acknowledgment: The authors thank Dr. Stanley Wearden, Nina Clovis, Suzanne Smith, and Vincent Kish for their assistance.
REFERENCES 1. Beynnon BD, Meriam CM, Ryder SH, Fleming BC, Johnson RJ. The effect of screw insertion torque on tendons fixed with spiked washers. Am J Sports Med 1998;4:536-539. 2. Almekinders LC, Chiavetta JB, Clarke JP. Radiological evaluation of anterior cruciate ligament graft failure with special reference to tibial tunnel placement. Arthroscopy 1998;2:206211. 3. Magen HE, Howell SM, Hull ML. Structural properties of six fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med 1999;1:19-23.
Purpose: As the use of soft-tissue anterior cruciate ligament (ACL) grafts, including hamstring grafts, has become more prominent and the benefits of aggressive rehabilitation have become clear, maximizing fixation with screw and spiked washers is important. Bicortical fixation may be superior. We were concerned about potential neurovascular risks and designed this study to define the posterior neurovasculature structures at risk when drilling for bicortical tibial screw fixation during ACL reconstruction. Type of Study: Consecutive sample. Methods: We placed the tibial tunnel arthroscopically in 10 cadaveric knees using a standard tibial drill guide. Accurate tibial tunnel position was documented in each knee by lateral radiograph. A 4.5-mm bicortical drill hole was placed perpendicular to the tibial surface 1 cm distal to the tibial tunnel. The distances from the posterior tibial drill exit point to nearby neurovascular structures were measured with a caliper. Results: The closest structure to the exit point was the bifurcation of the popliteal artery/vein (11.4 ⫾ 0.6 mm; range, 8.4 to 14.0 mm). The next closest was the anterior tibial vein (11.7 ⫾ 1.6 mm; range, 3.5 to 22.8 mm). The closest any individual hole came to a neurovascular structure was 3.5 mm from the anterior tibial vein. Conclusions: Bicortical drilling for fixation of soft tissue grafts appears reasonably safe. The structures at greatest risk for injury are the bifurcation of the popliteal artery/vein and the anterior tibial vein. Key Words: ACL graft—Fixation—Neurovascular—Complication—Drilling—Bicortical.
T
endon to bone fixation using a screw and spiked washer construct has been successfully used in many orthopedic procedures, including anterior cruciate ligament (ACL) reconstruction. Methods of graft fixation in ACL reconstruction have received intense scrutiny to reach goals including high load to failure, resistance to cyclic loading, ease of clinical use, and avoidance of potential complications. As the use of soft tissue ACL grafts, including hamstring grafts, has become more prominent and the benefits of aggressive
rehabilitation have become clear, maximizing fixation with screw and spiked washers is an important consideration. Insertion torque has been shown to correlate with load to failure.1 Because of our clinical experience, which suggests greater torque is possible using bicortical fixation of the tibial screw and spiked washer, we designed this study to evaluate the safety of bicortical drilling for placement of the tibial screw.
METHODS
From the Department of Orthopedics, West Virginia University, Morgantown (W.R.P.); and Marshall University School of Medicine, Huntington, West Virginia (S.S.K.), U.S.A. Address correspondence and reprint requests to William R. Post, M.D., Section of Sports Medicine and Shoulder Surgery, Department of Orthopedics, West Virginia University, PO Box 9196, Morgantown, WV 26506-9196, U.S.A. E-mail: [email protected] © 2001 by the Arthroscopy Association of North America 0749-8063/01/1703-2460$35.00/0 doi:10.1053/jars.2001.21539
244
Ten fresh-frozen cadaveric knees (midtibia/midfemur) were selected for study. The ACL was resected arthroscopically. A tibial drill guide (Arthrex, Naples, Fl; model AR-1866, AR-1867) was inserted through the medial parapatellar portal referencing the posterior cruciate ligament, with the angle set at 60° and a 2.4-mm guidewire was drilled. The drill guide was removed and a 9-mm cannulated drill was used to create the tibial tunnel, exactly as would be done
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 3 (March), 2001: pp 244 –247
NEUROVASCULAR RISK OF BICORTICAL TIBIAL DRILLING during routine ACL reconstruction. Soft tissue was cleared from the tibia just distal to the tibial tunnel. One centimeter distal to the tibial tunnel, we drilled a bicortical 4.5-mm hole perpendicular to the tibial surface using a custom cylindrical drill guide to assure perpendicular placement as is our practice clinically (Fig 1). Lateral knee radiographs in full extension were made with a 9-mm stainless steel rod in the tibial tunnel and a 4.5-mm drill bit in place in the tibia to document accurate tunnel placement. We measured the tunnel position by measuring the distances from the center of the steel rod at the level of the tibial plateau to the anterior and posterior borders of the plateau. The anterior/posterior percentage was calculated to document tunnel location per the technique of Almekinders et al.2 (Fig 2). The skin and subcutaneous layers were dissected from the posterior leg, protecting the deep fascia and neurovascular structures. The gastrocnemius was bisected longitudinally and reflected medially and laterally. Starting inferiorly, the soleus was reflected lat-
245
erally, taking care to free it from the neurovascular bundle. We were careful to not disturb the position of the neurovascular structures. At this point, the 4.5-mm drill bit was reinserted anteriorly. The posterior drill exit site was identified. Using a digital caliper (Mitutoyo U.K. Ltd., Hampshire, England, Model CD-6“B) we measured the shortest distance from the edge of the drill exit hole to 4 different structures: the trunk of the bifurcation of the popliteal artery or vein (whichever lay closer), the anterior tibial vein, the anterior tibial artery, and the peroneal nerve. Neurovascular structures were carefully examined for evidence of any damage that might have occurred from drilling, then all soft tissue was stripped from the tibiae. To define the consistency of tibial drill hole placement, we measured the shortest distance between the edge of the drill exit hole and knee joint capsule, lateral tibial border, and tibial articular surface. Anteriorly, we measured the shortest distance from the edge of the tibial tunnel to the tibial articular surface and the tibial midline. We also measured from the drill entry hole to the lateral border of the tibial tuberosity and to tibial articular surface. Lastly, drill hole depth was measured. Each measurement was taken 3 times, resetting the caliper to zero before each measurement. The caliper was used such that each measurement was determined before observing the numeric display to avoid observer bias. Readings were averaged for each individual specimen. Average distances were calculated for the study group. Intraclass correlation coefficients were calculated for all measurements. RESULTS Radiographic Tibial Tunnel Position The mean distance from the anterior border of the tibial plateau to the center of the tibial tunnel was 47% (SD, 4.9) of the anterior to posterior length of the tibial plateau. Anatomic Observations
FIGURE 1. Anterolateral and cross-sectional views of 3.5 mm drill bit pathway and tibial tunnel position.
There were no apparent injuries or anomalies involving any neurovascular structures. In 9 of 10 specimens, the drill exit hole was inferior to the tibiofibular joint capsule. One drill penetrated the joint capsule. Nine drill exit points were superior to the bifurcation of the popliteal artery/vein into the anterior and posterior tibial artery/vein. The lone exception here was 1 specimen in which the exit was almost directly anterior to the anterior tibial vein (3.5 mm). This was the closest that any drill hole came to a
246
W. R. POST AND S. S. KING
FIGURE 2. Lateral radiograph indicating typical anteroposterior position of the 9-mm tibial tunnel. Measurements indicated are (A) total anterior-posterior tibial plateau length, (B) anterior tibial plateau border to the center of the proximal tibial tunnel, and (C) posterior tibial plateau border to the center of the proximal tibial tunnel.
Magen et al.3 recently studied several techniques of screw and spiked washer fixation and found higher yield loads and less slippage compared with interference fixation. All the techniques they studied specified drilling of the posterior tibial cortex for fixation. Because we believe that bicortical fixation is likely to produce greater torque and superior fixation, we desired to study the anatomic position of the drill exit site relative to nearby neurovascular structures. We are unaware of reports of neurovascular injury secondary to placement of a tibial fixation drill hole. Nonetheless, because of concern regarding the possibility of neurovascular injury with bicortical drilling for tibial ACL graft fixation, we measured the distance from the exit hole to nearby neurovascular structures. We found the closest neurovascular structures were, on average, more than 1 cm from the drill exit site. However, 1 drill hole emerged only 3.5 mm from the tibial vein (but did not injure the vein). In this 1 case, had the drill plunged deeply it would have touched and possibly injured the anterior tibial vein. In the other 9 specimens, had the drill plunged deeply from the exit hole, continuing deeper would not have placed the drill tip in contact with neurovascular structures. We found slight variation in the starting point for tibial tunnel placement but minimal variation of the articular entrance of the tibial tunnel, a value that met accepted radiographic standards. We believe this represents a realistic variation for an experienced ACL surgeon. Based on our data and clinical experience, we believe bicortical tibial fixation screws can be placed safely, but surgeons must remember to be particularly cautious when perforating the posterior
neurovascular structure. Average measurements taken from the surface of the exit hole to the structures in question and tibial drill hole depth are summarized in Table 1. The closest structure, on average, to the drill exit hole was the trunk of the bifurcation (average, 11.4 mm). Next closest was the anterior tibial vein (average, 11.7 mm) with mean drill hole depth being 34.5 mm (Fig 3). DISCUSSION Tibial screw and spiked washer fixation has been used with cancellous screws (without drilling the posterior cortex) and with bicortical fixation. Many of the commercially available screws are cancellous screws by design. Recently, fixation strength has been described to correlate with the degree of torque applied.1
FIGURE 3. Actual drill hole exit sites referencing the lateral border and the posterior articular surface of a typical tibia.
NEUROVASCULAR RISK OF BICORTICAL TIBIAL DRILLING
247
TABLE 1. Measurements to Neurovascular Structures, Bony Landmarks, and Drill Depth Structure Drill exit to: Popliteal artery/vein bifurcation Anterior tibial vein Anterior tibial artery Peroneal nerve Joint capsule Lateral tuberosity border Posterior articular surface Drill entry to: Articular surface Tibial mid-line Lateral tibial border Tunnel to: Articular surface Tibial mid-line Tibial drill hole depth
Mean (mm)
Std Error⫾
Range (mm)
Intraclass Correlation
11.4 11.7 16.3 34.1 11.5 6.7 35.1
0.6 1.6 1.7 1.6 1.18 1.14 1.57
8.4-14.0 3.5-22.8 9.8-26.1 24.6-41.5 17.5-5.5 0.0-10.9 27.4-43.2
0.745 0.939 0.957 0.918 N/A* 0.966 0.864
39.5 28.0 35.5
1.74 1.04 2.7
30.4-46.4 22.6-32.8 31.5-41.6
0.736 0.988 0.987
23.7 28.0 34.5
2.29 1.19 1.48
15.4-39.3 20.0-33.5 26.4-41.4
0.727 0.993 0.999
* Not available; only 1 measurement/specimen taken.
tibial cortex. Given the proximity of neurovascular structures posteriorly, it might be wise to design a screw with a blunt tip to decrease the possibility of iatrogenic injury. Although not addressed in the current study, it also makes sense that it might be safer to drill this tunnel with the knee flexed to relax any potential tension in the posterior neurovascular structures. Based on this anatomic study, bicortical screw and spiked washer fixation of soft tissue ACL grafts appears to be relatively safe. The structures at greatest risk would be the bifurcations of the popliteal artery and vein and also the anterior tibial vein. Variations in anatomy and surgical technique are possible and we recommend care in drilling through the posterior cortex.
Acknowledgment: The authors thank Dr. Stanley Wearden, Nina Clovis, Suzanne Smith, and Vincent Kish for their assistance.
REFERENCES 1. Beynnon BD, Meriam CM, Ryder SH, Fleming BC, Johnson RJ. The effect of screw insertion torque on tendons fixed with spiked washers. Am J Sports Med 1998;4:536-539. 2. Almekinders LC, Chiavetta JB, Clarke JP. Radiological evaluation of anterior cruciate ligament graft failure with special reference to tibial tunnel placement. Arthroscopy 1998;2:206211. 3. Magen HE, Howell SM, Hull ML. Structural properties of six fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med 1999;1:19-23.