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 interfer- ence screw fixation. Key Words: Anterior cruciate ligament reconstruction-- Tibial tunnel--Graft placement--Graft fixation.

E ndoscopic anterior cruciate ligament (ACL) recon- struction with bone-patellar tendon-bone autograft

has become a popular orthopaedic procedure. Unfortu- nately, many technical considerations must be incorpo- rated into the surgical technique to avoid some of the "per i l s" that can lead to unsatisfactory results.~ Recent emphasis on roof impingement has led to new recom- mendations for more posteriorly directed tibial tunnel placement. 2'3 Tibial tunnel length is an equally im- portant 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-

From the Uniformed Services University of the Health Sciences, and the United States Air Force Academy, Colorado Springs, Colo- rado, 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 u s e .

0749-8063/96/1201-123850.00/0

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 in- terference 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 prepa- ration 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 pos- teromedial "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 mm in front of the posterior cruciate ligament. 8 Second, the

124 Arthroscopy: The Journal o f Arthroscopic and Related Surgery, Vol 12, No 1 (February), 1996: pp 124-126

THE " N 4- 7 RULE" 125

. . . . . "

I ' i i t /

al i

FIG 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.)

extraar t icular por t ion of the guide must be pos i t ioned midway be tween the apex of the t ibial tubercle and the pos te romedia l border o f the t ibia 9 (Fig 1). Final ly , the f i n a l guide must be set to the correct angle (de- scr ibed below) and or iented such that the " a r m " of the guide is perpendicular to the t ibia and paral le l to the ground (with the knee flexed 90°). (Fig 2).

The inter tendinous por t ion of the graft is measured (in ram), and ass igned the value " N " (Fig 3). Seven

t

FIg 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 added to this number, and the tibial guide angle is set based on the sum (N + 7). For example , if the distance between the bone plugs of the harvested bone- patel lar tendon-bone graft is 48 mm, the t ibial guide angle is set at 55 °, and the t ibial tunnel is dr i l led based on the guidel ines out l ined above.

The femoral tunnel is prepared using endoscopic techniques, leaving a 1- to 2 - ram poster ior cort ical shell, and the graft is pos i t ioned and fixed in the femo-

Tibial Plug Patella Plug

- - i

. . . . . . . a , - 2 - ~ o " - . - , ~ , ~ o . . . . .

FIG 3. The distance between the two bone plugs is carefully mea- sured (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 D. T. H I N K I N

ral tunnel with an anter ior ly p laced interference screw. Using this technique, the t ibial bone plug wil l consis- tently be located within the t ibial tunnel, a l lowing in- terference screw fixation.

D I S C U S S I O N

A n appropr ia te ly sized t ibial tunnel is an essential feature o f endoscopic A C L reconstruct ion. A tunnel that is too short wil l resul t in graft extrusion, necessi tat- ing supplementa l f ixation techniques. I f the tunnel is p laced too distal ly, it may be imposs ib le to reach the femoral pi lot hole through the t ibial hole with a guide pin and reamer or interference screw fixation may be difficult. 4 Appl ica t ion o f the " N + 7 R u l e " has re-

sulted in consis tent ly accurate tunnel lengths. W e have had to use supplementa l f ixation techniques in only one case f rom more than 100 endoscopic A C L recon- structions since we have begin apply ing this s imple guideline. A review of the technical aspects o f this par t icular case suggested that the " a r m " o f the t ibial guide was not correct ly p laced perpendicu lar to the tibia. This highl ights the impor tance o f correct orienta- t ion o f the guide when apply ing the " N + 7 Ru le . "

REFERENCES

1. Miller MD, Johnson DL, Fu FH. Anterior cruciate ligament re- construction: Autografi and allografl: Pearls and perils. Video- tape, VT-24131. Rosemont, IL: American Academy of Orthopae- dic Surgeons, 1994.

2. Yam NC, Daniel DM, Pennar D. The effects of tibial attachment site on graft impingement in anterior cruciate ligament recon- struction. 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 inter- ference fixation: Technical note. Arthroscopy 1993;9:228-230.

6. Jackson DW, Gasser SI. Tibia1 tunnel placement in ACL recon- struction. 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; 1 1:275-288.

9. Olson EJ, Fu FH, Hamer CD, et al. Towards optimal tibial tunnel placement in endoscopic anterior cruciate ligament reconstruc- tion. Poster Exhibit A57, American Academy of Orthopaedic Surgeons, San Francisco, CA, 1993.