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The “N + 7 Rule” for tibial tunnel placement in endoscopic anterior cruciate ligament reconstruction
Technical Note
The "N + 7 Rule" for Tibial Tunnel Placement in Endoscopic Anterior Cruciate Ligament Reconstruction Maj Mark D. Miller, M.D., MC, USAF, and Lt Col Daniel T. Hinkin, M.D., MC, USAF
Summary: Tibial tunnel placement during endoscopic anterior cruciate ligament (ACL) reconstruction has received increased emphasis in the recent literature. Appropriate tunnel length is a critical technical consideration. A tunnel that is too short results in graft extrusion, necessitating supplemental fixation techniques. A tunnel that is too long may make distal fixation and femoral tunnel placement difficult. A simple rule is proposed that allows for correct tunnel length and allows placement of the bone plug consistently within the tibial tunnel, allowing interference screw fixation. Key Words: Anterior cruciate ligament reconstruction-Tibial tunnel--Graft placement--Graft fixation.
ndoscopic anterior cruciate ligament (ACL) reconstruction with bone-patellar tendon-bone autograft has become a popular orthopaedic procedure. Unfortunately, many technical considerations must be incorporated into the surgical technique to avoid some of the "perils" that can lead to unsatisfactory results.~ Recent emphasis on roof impingement has led to new recommendations for more posteriorly directed tibial tunnel placement. 2'3 Tibial tunnel length is an equally important consideration. With the endoscopic technique, longer tibial tunnels are required to keep the patellar bone plug contained within the bony tibial tunnel. 4 Failure to account for this results in a graft that is "too long," necessitating some form of fixation other than interference screw fixation. According to one group of surgeons, 5'6 required fibial tunnel length can be calcu-
E
From the Uniformed Services University of the Health Sciences, and the United States Air Force Academy, Colorado Springs, Colorado, U.S.A. Address correspondence and reprint requests to Maj Mark D. Miller, M.D., MC, USAF, 14515 River Oaks Dr, Colorado Springs, CO 80921, U.S.A. This is a US government work. There are no restrictions on its use.
0749-8063/96/1201-123850.00/0
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lated and the length of the tibial tunnel adjusted by moving the starting point proximally or distally based on measurements from a calibrated guide. However, we have found these calculations to be cumbersome and difficult to consistently interpret intraoperatively. In an effort to simplify these considerations, an easy and consistently reproducible technique was developed to ensure appropriate tibial tunnel length, allowing interference fixation in all cases.
SURGICAL T E C H N I Q U E The technique of single-incision endoscopic ACL reconstruction has been well described previously. 4'7 Several technical points regarding tibial tunnel preparation must be carefully followed to successfully apply the " N + 7 Rule." First, the intraarticular entry point of the tibial tunnel should be centered within the posteromedial "footprint" of the native ACL insertion. This is practically done by placing the tibial guide immediately in front of the posterior cruciate ligament slightly medial to midline. Ideally, the tibial tunnel guide pin should enter the joint approximately 7 m m in front of the posterior cruciate ligament. 8 Second, the
Arthroscopy: The Journal o f Arthroscopic and Related Surgery, Vol 12, No 1 (February), 1996: p p 124-126
THE " N 4- 7 R U L E "
125
t
.....
'
ii
"
t
/
I
al i
F I g 2. The tibial tunnel guide should be set for the appropriate slope (open arrow) and should be perpendicular to the tibia (flexed 90°) and parallel with the floor (shaded arrow).
is a d d e d to this number, and the tibial guide angle is set based on the sum (N + 7). F o r example, if the distance between the bone plugs o f the harvested bonepatellar t e n d o n - b o n e graft is 48 mm, the tibial guide angle is set at 55 °, and the tibial tunnel is drilled b a s e d on the guidelines outlined above. The femoral tunnel is p r e p a r e d using endoscopic techniques, leaving a 1- to 2 - r a m posterior cortical shell, and the graft is positioned and fixed in the femoFIG 1. The extraarticular portion of the tibial tunnel should begin midway between the apex of the tibial tubercle and the posteromedial border of the tibia. The arrow demonstrates the ideal "starting point" for the tibial tunnel. Tunnels placed medial or lateral to this location will adversely affect femoral tunnel placement. (Drawing courtesy of Eric J. Olsen, M.D.)
Patella Plug
Tibial Plug
- -
extraarticular portion o f the guide must b e p o s i t i o n e d m i d w a y b e t w e e n the apex o f the tibial tubercle and the p o s t e r o m e d i a l b o r d e r o f the tibia 9 (Fig 1). Finally, the f i n a l guide m u s t be set to the correct angle (described below) and oriented such that the " a r m " o f the guide is p e r p e n d i c u l a r to the tibia and parallel to the ground (with the knee flexed 90°). (Fig 2). The intertendinous portion o f the graft is m e a s u r e d (in ram), and assigned the value " N " (Fig 3). Seven
.
.
.
.
.
.
.
a ,-2-
~o"
-.-, ~ ,
i
~o.
. . . .
FIG 3. The distance between the two bone plugs is carefully measured (in mm) and assigned the value "N." Seven is added to this value, and the guide is set to this angle.
126
M. D. M I L L E R A N D
ral tunnel with an anteriorly p l a c e d interference screw. U s i n g this technique, the tibial b o n e plug will consistently be located within the tibial tunnel, allowing interference screw fixation. DISCUSSION A n a p p r o p r i a t e l y sized tibial tunnel is an essential feature o f e n d o s c o p i c A C L reconstruction. A tunnel that is too short will result in graft extrusion, necessitating supplemental fixation techniques. I f the tunnel is p l a c e d too distally, it m a y be i m p o s s i b l e to reach the femoral pilot hole through the tibial hole with a guide pin and r e a m e r or interference screw fixation m a y b e difficult. 4 A p p l i c a t i o n o f the " N + 7 R u l e " has resulted in consistently accurate tunnel lengths. W e have had to use s u p p l e m e n t a l fixation techniques in only one case from m o r e than 100 e n d o s c o p i c A C L reconstructions since w e have begin a p p l y i n g this simple guideline. A r e v i e w o f the technical aspects o f this particular case suggested that the " a r m " o f the tibial guide was not correctly p l a c e d p e r p e n d i c u l a r to the tibia. This highlights the i m p o r t a n c e o f correct orientation o f the guide when a p p l y i n g the " N + 7 R u l e . "
D. T. H I N K I N
REFERENCES 1. Miller MD, Johnson DL, Fu FH. Anterior cruciate ligament reconstruction: Autografi and allografl: Pearls and perils. Videotape, VT-24131. Rosemont, IL: American Academy of Orthopaedic Surgeons, 1994. 2. Yam NC, Daniel DM, Pennar D. The effects of tibial attachment site on graft impingement in anterior cruciate ligament reconstruction. Am J Sports Med 1992;20:217-220. 3. Howell SM, Clark JA. Tibial tunnel placement in anterior cruciate ligament reconstructions and graft impingement. Clin Orthop 1992;283:187-195. 4. Christian CA, Indelicato PA. Allograft anterior cruciate ligament reconstruction with patellar tendon: An endoscopic technique. Oper Tech Sports Med 1993; 1:50-57. 5. Kenna B, Simon TM, Jackson DW, Kurzwell PR. Endoscopic ACL reconstruction: A technical note on tunnel length for interference fixation: Technical note. Arthroscopy 1993;9:228-230. 6. Jackson DW, Gasser SI. Tibia1 tunnel placement in ACL reconstruction. Arthroscopy 1994; 10:124-131. 7. Beck CL, Paulos LE, Rosenberg TD. Anterior cruciate ligament reconstruction with endoscopic technique. Oper Tech Orthop 1992;2:86-98. 8. Morgan CD, Kalman VR, Grawl DM. Definitive landmarks for reproducible tibial tunnel placement in anterior cruciate ligament reconstruction. Arthroseopy 1995;11:275-288. 9. Olson EJ, Fu FH, Hamer CD, et al. Towards optimal tibial tunnel placement in endoscopic anterior cruciate ligament reconstruction. Poster Exhibit A57, American Academy of Orthopaedic
Surgeons, San Francisco, CA, 1993.
The "N + 7 Rule" for Tibial Tunnel Placement in Endoscopic Anterior Cruciate Ligament Reconstruction Maj Mark D. Miller, M.D., MC, USAF, and Lt Col Daniel T. Hinkin, M.D., MC, USAF
Summary: Tibial tunnel placement during endoscopic anterior cruciate ligament (ACL) reconstruction has received increased emphasis in the recent literature. Appropriate tunnel length is a critical technical consideration. A tunnel that is too short results in graft extrusion, necessitating supplemental fixation techniques. A tunnel that is too long may make distal fixation and femoral tunnel placement difficult. A simple rule is proposed that allows for correct tunnel length and allows placement of the bone plug consistently within the tibial tunnel, allowing interference screw fixation. Key Words: Anterior cruciate ligament reconstruction-Tibial tunnel--Graft placement--Graft fixation.
ndoscopic anterior cruciate ligament (ACL) reconstruction with bone-patellar tendon-bone autograft has become a popular orthopaedic procedure. Unfortunately, many technical considerations must be incorporated into the surgical technique to avoid some of the "perils" that can lead to unsatisfactory results.~ Recent emphasis on roof impingement has led to new recommendations for more posteriorly directed tibial tunnel placement. 2'3 Tibial tunnel length is an equally important consideration. With the endoscopic technique, longer tibial tunnels are required to keep the patellar bone plug contained within the bony tibial tunnel. 4 Failure to account for this results in a graft that is "too long," necessitating some form of fixation other than interference screw fixation. According to one group of surgeons, 5'6 required fibial tunnel length can be calcu-
E
From the Uniformed Services University of the Health Sciences, and the United States Air Force Academy, Colorado Springs, Colorado, U.S.A. Address correspondence and reprint requests to Maj Mark D. Miller, M.D., MC, USAF, 14515 River Oaks Dr, Colorado Springs, CO 80921, U.S.A. This is a US government work. There are no restrictions on its use.
0749-8063/96/1201-123850.00/0
124
lated and the length of the tibial tunnel adjusted by moving the starting point proximally or distally based on measurements from a calibrated guide. However, we have found these calculations to be cumbersome and difficult to consistently interpret intraoperatively. In an effort to simplify these considerations, an easy and consistently reproducible technique was developed to ensure appropriate tibial tunnel length, allowing interference fixation in all cases.
SURGICAL T E C H N I Q U E The technique of single-incision endoscopic ACL reconstruction has been well described previously. 4'7 Several technical points regarding tibial tunnel preparation must be carefully followed to successfully apply the " N + 7 Rule." First, the intraarticular entry point of the tibial tunnel should be centered within the posteromedial "footprint" of the native ACL insertion. This is practically done by placing the tibial guide immediately in front of the posterior cruciate ligament slightly medial to midline. Ideally, the tibial tunnel guide pin should enter the joint approximately 7 m m in front of the posterior cruciate ligament. 8 Second, the
Arthroscopy: The Journal o f Arthroscopic and Related Surgery, Vol 12, No 1 (February), 1996: p p 124-126
THE " N 4- 7 R U L E "
125
t
.....
'
ii
"
t
/
I
al i
F I g 2. The tibial tunnel guide should be set for the appropriate slope (open arrow) and should be perpendicular to the tibia (flexed 90°) and parallel with the floor (shaded arrow).
is a d d e d to this number, and the tibial guide angle is set based on the sum (N + 7). F o r example, if the distance between the bone plugs o f the harvested bonepatellar t e n d o n - b o n e graft is 48 mm, the tibial guide angle is set at 55 °, and the tibial tunnel is drilled b a s e d on the guidelines outlined above. The femoral tunnel is p r e p a r e d using endoscopic techniques, leaving a 1- to 2 - r a m posterior cortical shell, and the graft is positioned and fixed in the femoFIG 1. The extraarticular portion of the tibial tunnel should begin midway between the apex of the tibial tubercle and the posteromedial border of the tibia. The arrow demonstrates the ideal "starting point" for the tibial tunnel. Tunnels placed medial or lateral to this location will adversely affect femoral tunnel placement. (Drawing courtesy of Eric J. Olsen, M.D.)
Patella Plug
Tibial Plug
- -
extraarticular portion o f the guide must b e p o s i t i o n e d m i d w a y b e t w e e n the apex o f the tibial tubercle and the p o s t e r o m e d i a l b o r d e r o f the tibia 9 (Fig 1). Finally, the f i n a l guide m u s t be set to the correct angle (described below) and oriented such that the " a r m " o f the guide is p e r p e n d i c u l a r to the tibia and parallel to the ground (with the knee flexed 90°). (Fig 2). The intertendinous portion o f the graft is m e a s u r e d (in ram), and assigned the value " N " (Fig 3). Seven
.
.
.
.
.
.
.
a ,-2-
~o"
-.-, ~ ,
i
~o.
. . . .
FIG 3. The distance between the two bone plugs is carefully measured (in mm) and assigned the value "N." Seven is added to this value, and the guide is set to this angle.
126
M. D. M I L L E R A N D
ral tunnel with an anteriorly p l a c e d interference screw. U s i n g this technique, the tibial b o n e plug will consistently be located within the tibial tunnel, allowing interference screw fixation. DISCUSSION A n a p p r o p r i a t e l y sized tibial tunnel is an essential feature o f e n d o s c o p i c A C L reconstruction. A tunnel that is too short will result in graft extrusion, necessitating supplemental fixation techniques. I f the tunnel is p l a c e d too distally, it m a y be i m p o s s i b l e to reach the femoral pilot hole through the tibial hole with a guide pin and r e a m e r or interference screw fixation m a y b e difficult. 4 A p p l i c a t i o n o f the " N + 7 R u l e " has resulted in consistently accurate tunnel lengths. W e have had to use s u p p l e m e n t a l fixation techniques in only one case from m o r e than 100 e n d o s c o p i c A C L reconstructions since w e have begin a p p l y i n g this simple guideline. A r e v i e w o f the technical aspects o f this particular case suggested that the " a r m " o f the tibial guide was not correctly p l a c e d p e r p e n d i c u l a r to the tibia. This highlights the i m p o r t a n c e o f correct orientation o f the guide when a p p l y i n g the " N + 7 R u l e . "
D. T. H I N K I N
REFERENCES 1. Miller MD, Johnson DL, Fu FH. Anterior cruciate ligament reconstruction: Autografi and allografl: Pearls and perils. Videotape, VT-24131. Rosemont, IL: American Academy of Orthopaedic Surgeons, 1994. 2. Yam NC, Daniel DM, Pennar D. The effects of tibial attachment site on graft impingement in anterior cruciate ligament reconstruction. Am J Sports Med 1992;20:217-220. 3. Howell SM, Clark JA. Tibial tunnel placement in anterior cruciate ligament reconstructions and graft impingement. Clin Orthop 1992;283:187-195. 4. Christian CA, Indelicato PA. Allograft anterior cruciate ligament reconstruction with patellar tendon: An endoscopic technique. Oper Tech Sports Med 1993; 1:50-57. 5. Kenna B, Simon TM, Jackson DW, Kurzwell PR. Endoscopic ACL reconstruction: A technical note on tunnel length for interference fixation: Technical note. Arthroscopy 1993;9:228-230. 6. Jackson DW, Gasser SI. Tibia1 tunnel placement in ACL reconstruction. Arthroscopy 1994; 10:124-131. 7. Beck CL, Paulos LE, Rosenberg TD. Anterior cruciate ligament reconstruction with endoscopic technique. Oper Tech Orthop 1992;2:86-98. 8. Morgan CD, Kalman VR, Grawl DM. Definitive landmarks for reproducible tibial tunnel placement in anterior cruciate ligament reconstruction. Arthroseopy 1995;11:275-288. 9. Olson EJ, Fu FH, Hamer CD, et al. Towards optimal tibial tunnel placement in endoscopic anterior cruciate ligament reconstruction. Poster Exhibit A57, American Academy of Orthopaedic
Surgeons, San Francisco, CA, 1993.