- Email: [email protected]
Resident’s ridge: assessing the cortical thickness of the lateral wall and roof of the intercondylar notch
Resident’s Ridge: Assessing the Cortical Thickness of the Lateral Wall and Roof of the Intercondylar Notch Mark R. Hutchinson, M.D., and Stephen A. Ash, M.D.
Purpose: The purpose of this study was to better delineate the anatomy of “resident’s ridge,” a term coined by William Clancy Jr., M.D., to describe the raised bony landmark commonly visualized just anterior to the femoral attachment of the anterior cruciate ligament (ACL). This landmark can mislead the novice surgeon into misplacing the femoral tunnel of ACL reconstructions. Type of Study: Cadaveric anatomic study. Methods: Ten human distal femurs harvested from embalmed specimens were fixed, sectioned, and analyzed for the presence and descriptive characteristics of resident’s ridge. A single, blinded examiner evaluated slope, cortical thickness at 4 sites, and the presence or absence of a distinct ridge relative to the attachment of the ACL. Results: A defined resident’s ridge was present in 9 of 10 specimens. This was directly associated with a change in slope of the intracondylar roof in the same 9 of 10 patients. The mean cortical thickness at the ACL attachment site was 1.6 mm. This was thicker than at resident’s ridge (mean, 0.90 mm), the cartilage-intercondylar notch junction (mean, 0.96 mm), and a point midway between the ACL attachment and the cartilage–intercondylar notch junction (mean, 0.90 mm). Conclusions: The phenomenon of “resident’s ridge” is accounted for by a distinctive change in slope of the femoral notch roof that occurs just anterior to the femoral attachment of the ACL. The density change apparent at the time of notchplasty is probably caused by the transition between normal cortical thickness just anterior to the ACL and the cortical thickness of the ACL attachment. No distinctive increased cortical thickness can be identified as “resident’s ridge.” Key Words: Resident’s ridge— Anterior cruciate ligament—Anatomic landmarks.
A
ccurate and anatomic tunnel placements are essential to the success of reconstruction of the anterior cruciate ligament (ACL). On the femoral side, ideal tunnel placement in the sagittal plane has been described as in the posterior quartile. In the coronal plane, the goal is commonly between the 1- and 2-o’clock positions for left knees and the 10- and 11-o’clock positions for right knees. Vertical tunnel placement may control anteroposterior stability but offers poor rotational control. Anterior tunnel place-
From the Sports Medicine and Human Performance Center, Department of Orthopaedics, University of Illinois at Chicago, Chicago, Illinois, U.S.A. Address correspondence and reprint requests to Mark R. Hutchinson, M.D., Associate Professor of Orthopaedics and Sports Medicine, 209 Medical Sciences South, M/C 844, 901 South Wolcott, Chicago, IL 60612, U.S.A. E-mail: [email protected] © 2003 by the Arthroscopy Association of North America 0749-8063/03/1909-3266$30.00/0 doi:10.1016/S0749-8063(03)00809-0
ment leads to an increased risk of failure. The graft is overtightened in a flexed position, and either the patient loses extension or the graft stretches to allow extension. Femoral guides have been designed to reduce the risk of anterior femoral tunnel placement; nonetheless, tunnel malposition continues to be one of the most common causes of reconstruction failure. One potential reason for this is failure to clearly identify the true over-the-back position on the femur in the posterior aspect of the notch. A relative prominence or change in slope is routinely present 3 quarters of the way back on the roof to lateral border of the notch, which can easily be mistaken for the true-over-the back position. William G. Clancy, Jr., M.D., originally coined the term “resident’s ridge” (Clancy, personal communication, 1998). “. . . when residents were beginning to learn how to do our anterior cruciate ligament reconstruction, they did not adequately debride the entire
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 19, No 9 (November), 2003: pp 931-935
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M. R. HUTCHINSON AND S. A. ASH
posterior aspect of the lateral femoral condyle and assumed they were looking at the most posterior edge when they saw the ‘resident’s ridge.’” The resident’s ridge is a vertical to transverse ridge approximately 75% posterior from the anterior aspect of the femur on the lateral femoral condyle with the knee flexed to 90° (Fig 1). Mistaking resident’s ridge for the true overthe-back position leads to anterior tunnel placement and, therefore, premature failure of reconstructions. When performing arthroscopic notchplasties, we noticed that resident’s ridge appeared to be harder than the bone of the lateral intercondylar notch anterior to it. This change frequently caused the arthroscopic burr to jump or skip. We hypothesized that the exact anatomy that accounted for resident’s ridge may be because of a cortical thickening that served as a bumper to the anterior cruciate ligament. This would explain both the ridge and our arthroscopic findings when burring the ridge. The purpose of this study was, therefore, to characterize the anatomy of the lateral wall and roof of the intercondylar notch, to better define resident’s ridge, and provide an explanation for the arthroscopic findings during burring of this area. Increased awareness and knowledge should, in turn, allow the surgeon to make a reproducible femoral tunnel for ACL reconstruction. METHODS Ten human distal femurs were harvested from embalmed cadavers at the completion of their use in the University of Illinois at Chicago medical school anatomy laboratory. None had been previously dissected in the area of interest. The specimens were dehydrated in ethyl alcohol and cleared in xylene. They were infiltrated with 3 different viscosities of methylmethacrylate and then polymerized in methyl methacrylate. Cross-sections 2-mm thick were made using an Exakt cutting system diamond band-saw blade, 0.3-mm thick (Exakt Technologies, Oklahoma City, OK). The plane of the cross-sections was a slightly canted sagittal section selected by approximating the 11- or 1-o’clock position for the right or left femur, respectively, and then using a plane parallel to this. The cut specimens were surface stained with Paragon stain to distinguish cartilage and soft tissues from bone. The canted sagittal cross-sections were cut from medial to lateral based on the width of the ACL attachment. The cross-section judged to be in the center of the ACL attachment from medial to lateral was selected for measurement of the cortical thickness at the center of the ACL attachment, the anterior
FIGURE 1. (A) Arthroscopic view of right knee identifying resident’s ridge with a probe. Note the true over-the-back position posterior to the ridge. (B) Arthroscopic view of anterior placement of the femoral tunnel if resident’s ridge is used instead of the true over-the-back position. (C) Arthroscopic view of femoral guide in appropriate placement. Note the presence of a remnant of resident’s ridge and the old ACL stump.
condylar hyaline cartilage junction, a point midway between the ACL and the condylar cartilage junction, and resident’s ridge (usually noted 1 mm anterior to the anterior edge of the ACL attachment). This cross-
RESIDENT’S RIDGE
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TABLE 1. Cortical Thickness at Various Sites on the Roof of the Intracondylar Notch Specimen
Cartilage Junction
Midway Point
Resident’s Ridge
ACL Insertion
1 2 3 4 5 6 7 8 9 10 Mean SD
1.26 0.50 1.46 1.32 1.00 1.00 0.76 0.50 1.08 0.76 0.96 mm 0.31 mm
1.38 0.32 1.12 1.62 1.12 0.88 0.62 0.96 0.62 0.36 0.90 mm 0.40 mm
1.62 0.30 1.88 0.28 1.70 0.92 0.46 0.56 1.00 0.26 0.90 mm 0.60 mm
1.76 1.40 2.62 1.76 1.56 2.26 0.70 1.50 1.48 0.96 1.60 mm 0.51 mm
(Model No. 1048; American Optical, Southbridge, MA) with Olympus eyepieces (Olympus, Melville, NY) equipped with a calibrated measuring scale was used to measure the cortical thickness. Measurements were made to the nearest 0.02 mm. A randomized block design with the Tukey adjustment for multiple comparisons was used to analyze differences in site thickness (SAS/STAT v. 6.12 software for SAS Institute, Cary, NC). RESULTS
FIGURE 2. (A) Canted cross-sectional sagittal section of distal femur specimen with (B) accompanying line drawing. The solid stout arrow points to the change in slope of the notch roof that correlates with resident’s ridge. The acute curved arrow points to the hyaline cartilage on the anterior aspect of the femur in the trochlea. The 2 hollow arrows point to the cortical thickening beneath the attachment of the anterior cruciate ligament. The solid curved arrow references the insertion and fibers of the anterior cruciate ligament. The 3 triangular arrows point to the posterior capsule and its attachment on the posterior aspect of the femur.
section was also examined qualitatively for the presence or absence of a ridge anterior to the ACL attachment and the change of slope noted there (Fig 2). Any raised ridge of greater than 1 mm or change in slope of greater than 5° was considered consistent with the presence of a ridge. An American Optical microscope
The mean thickness of cortical bone was 0.96 mm at the cartilage-notch junction site, 0.90 mm at the midway point, 0.90 mm at resident’s ridge, and 1.60 mm at the ACL attachment site (Table 1). Multiple comparison testing between sites was performed. The only sites with a significant difference were related to the ACL attachment (Table 2). Although the standard deviation was relatively broad, specimen 5 was the only specimen in which any measurement (in this case resident’s ridge) exceeded the ACL attachment measurement. TABLE 2. Statistical Comparisons Between Sites Comparison
P
Cartilage junction v midway point Cartilage junction v resident’s ridge Cartilage junction v ACL insertion Midway point v ACL insertion Midway point v resident’s ridge Resident’s ridge v ACL insertion
.9790 .9735 .0016* .0006* 1.000 .0005*
*Statistically significant by randomized block design with Tukey adjustment for multiple comparisions.
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M. R. HUTCHINSON AND S. A. ASH
Observations regarding the contours of the crosssections near the anterior border of the ACL attachment were instructive. We defined a significant change in slope as greater than 5°. In 9 of 10 specimens, the lateral wall cortex appeared to change slope just anterior to the thickening of the ACL attachment site. This change in slope corresponded to what is commonly referred to as resident’s ridge. DISCUSSION Although some studies have documented the dimensions of the notch and femoral attachment site, none has evaluated the cortical thickness of the lateral wall and roof of the notch or the bony anatomy accounting for resident’s ridge.1-8 Surgical awareness of notch roof anatomy is important.9-10 A surgeon who inadvertently mistakes resident’s ridge for the true over-the-top position at the back of the intercondylar notch will create an unacceptably anterior femoral tunnel. Carson et al.10 noted that the most frequent surgical error causing ACL reconstruction failure is anterior femoral tunnel placement. Our findings document the anatomy of resident’s ridge and hopefully will assist surgeons in its recognition to avoid malposition of the femoral tunnel placement. During preparation of the intercondylar notch, many surgeons will use a burr to take down or flatten resident’s ridge to assure clear visualization and access to the over-the-top position. During this notch preparation, the burr will frequently jump at the apparently harder or thicker bone of resident’s ridge. Theoretically, this finding could be secondary to a number of factors. These factors include that resident’s ridge could have a thicker cortex and denser bone that would serve as an anterior bumper for the ACL (our original hypothesis); that the entire femoral attachment of the ACL could have a thicker cortex, with resident’s ridge representing only the leading edge of the more prominent cortex and dense bone; or that clinical observation of denser bone could be inaccurate and that no variation in the cortical thickness exists relative to the intercondylar notch, resident’s ridge, and the femoral ACL attachment. Our findings support the second of these possibilities. The femoral attachment of the ACL is the only region we measured with a significantly thicker cortex. It is thicker than resident’s ridge, the hyaline cartilage-intracondylar notch junction, and the point midway between the ACL attachment and the hyaline cartilage-intracondylar notch junction. Resident’s ridge, in and of itself, does not have a thicker cortex.
Variable cortical thickness is a challenge the surgeon must contend with to avoid erratic burr behavior. In addition, the change in slope evident in 90% of our specimens probably was a contributing factor. Potential limitations of this study include the selection of specimens and the relative broad standard deviation noted regarding cortical thickness measurements at each site. The variability of cortical thickness between specimens is probably tied to the relatively broad range of ages of the embalmed specimens chosen. One would reasonably expect that the younger specimens would reveal thicker cortices and denser bone; nonetheless, that the relationship of cortical thickness between sites should be significantly altered is unlikely. Osteopenia and osteoporosis are, in general, diffuse processes and should affect each site at relatively the same pace. More specifically, regardless of age, the relative increased cortical thickness at the ACL attachment site should be thicker than the resident’s ridge, the hyaline cartilage-intracondylar notch junction, and the point midway between the ACL attachment and the hyaline cartilage intracondylar notch junction. Indeed, despite the broad range of standard deviation of measurements between specimens, only 1 specimen (specimen No. 5) had any measurement that exceeded the cortical thickness measured at the ACL attachment. In conclusion, resident’s ridge is a change in slope or ridge located about 75% posteriorly on the roof and lateral wall of the intracondylar notch of the knee. It is located just anterior to the ACL attachment and anterior to the posterior limit of the intracondylar notch (the over-the-top position). Resident’s ridge is a consistent anatomic landmark identifiable in 9 of 10 knees we studied. Resident’s ridge represents the change in slope of the notch roof as well as a transitional thickness of cortex from notch to actual ACL attachment point. The term resident’s ridge was coined by William Clancy, Jr., M.D. The significance of resident’s ridge is that it can be misinterpreted as the posterior roof of the notch and can inadvertently lead to nonanatomic anterior placement of the femoral tunnel for ACL reconstruction. Special care when creating the femoral tunnel for ACL reconstruction should include an awareness of resident’s ridge. Carefully reducing the ridge with a burr will assist in confirming the true over-the-top position as the appropriate anatomic landmark. This should make accurate tunnel placement more reproducible. Finally, variable cortical thickness is a challenge the surgeon must face to avoid erratic burr behavior. The variation in cortical density
RESIDENT’S RIDGE is explained by the femoral attachment of the ACL itself and not the resident’s ridge. Acknowledgment: The authors thank Susan King, H.T. (A.S.C.P.), Robert Anderson, Ph.D., Luna Ghosh, M.D., Karen Ferrer, M.D., and Taran Bae, M.D., for their technical assistance.
REFERENCES 1. Anderson AF, Lipscomb AB, Liudahl KJ, et al. Analysis of the intercondylar notch by computed tomography. Am J Sports Med 1987;15:547-552. 2. Herzog RJ, Silliman JF, Hutton K, et al. Measurements of the intercondylar notch by plain film radiography and magnetic resonance imaging. Am J Sports Med 1994;22:204-210. 3. Muneta T, Takakuda K, Yamamoto H. Intercondylar notch width and its relation to the configuration and cross-sectional area of the anterior cruciate ligament. Am J Sports Med 1997; 25:60-72.
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4. Good L, Odensten M, Gillquist J. Intercondylar notch measurements with special reference to anterior cruciate ligament surgery. Clin Orthop 1991;263:185-189. 5. Houseworth SW, Mauro VJ, Mellon BA. The intercondylar notch in acute tears of the anterior cruciate ligament: A computer graphics study. Am J Sports Med 1987;15:221-224. 6. Mensch JS, Amstutz HC. Knee morphology as a guide to knee replacement. Clin Orthop 1975;112:231-241. 7. Shelbourne KD, Davis TJ, Klootwyk TE. The relationship between intercondylar notch width of the femur and the incidence of anterior cruciate ligament tears: A prospective study. Am J Sports Med 1998;26:402-408. 8. Girgis FG, Marshall JL, Al Mohajem ARS. The cruciate ligaments of the knee joint: Anatomical, functional, and experimental analysis. Clin Orthop 1975;106:216-231. 9. Dalton JD, Harner CD. Surgical techniques to correct nonanatomic femoral tunnels. Oper Tech Sports Med 1998;6:8390. 10. Carson EW, Simonian PT, Wickiewicz TL, et al. Revision anterior cruciate ligament reconstruction. Instr Course Lect 1998;47:361-368.
Purpose: The purpose of this study was to better delineate the anatomy of “resident’s ridge,” a term coined by William Clancy Jr., M.D., to describe the raised bony landmark commonly visualized just anterior to the femoral attachment of the anterior cruciate ligament (ACL). This landmark can mislead the novice surgeon into misplacing the femoral tunnel of ACL reconstructions. Type of Study: Cadaveric anatomic study. Methods: Ten human distal femurs harvested from embalmed specimens were fixed, sectioned, and analyzed for the presence and descriptive characteristics of resident’s ridge. A single, blinded examiner evaluated slope, cortical thickness at 4 sites, and the presence or absence of a distinct ridge relative to the attachment of the ACL. Results: A defined resident’s ridge was present in 9 of 10 specimens. This was directly associated with a change in slope of the intracondylar roof in the same 9 of 10 patients. The mean cortical thickness at the ACL attachment site was 1.6 mm. This was thicker than at resident’s ridge (mean, 0.90 mm), the cartilage-intercondylar notch junction (mean, 0.96 mm), and a point midway between the ACL attachment and the cartilage–intercondylar notch junction (mean, 0.90 mm). Conclusions: The phenomenon of “resident’s ridge” is accounted for by a distinctive change in slope of the femoral notch roof that occurs just anterior to the femoral attachment of the ACL. The density change apparent at the time of notchplasty is probably caused by the transition between normal cortical thickness just anterior to the ACL and the cortical thickness of the ACL attachment. No distinctive increased cortical thickness can be identified as “resident’s ridge.” Key Words: Resident’s ridge— Anterior cruciate ligament—Anatomic landmarks.
A
ccurate and anatomic tunnel placements are essential to the success of reconstruction of the anterior cruciate ligament (ACL). On the femoral side, ideal tunnel placement in the sagittal plane has been described as in the posterior quartile. In the coronal plane, the goal is commonly between the 1- and 2-o’clock positions for left knees and the 10- and 11-o’clock positions for right knees. Vertical tunnel placement may control anteroposterior stability but offers poor rotational control. Anterior tunnel place-
From the Sports Medicine and Human Performance Center, Department of Orthopaedics, University of Illinois at Chicago, Chicago, Illinois, U.S.A. Address correspondence and reprint requests to Mark R. Hutchinson, M.D., Associate Professor of Orthopaedics and Sports Medicine, 209 Medical Sciences South, M/C 844, 901 South Wolcott, Chicago, IL 60612, U.S.A. E-mail: [email protected] © 2003 by the Arthroscopy Association of North America 0749-8063/03/1909-3266$30.00/0 doi:10.1016/S0749-8063(03)00809-0
ment leads to an increased risk of failure. The graft is overtightened in a flexed position, and either the patient loses extension or the graft stretches to allow extension. Femoral guides have been designed to reduce the risk of anterior femoral tunnel placement; nonetheless, tunnel malposition continues to be one of the most common causes of reconstruction failure. One potential reason for this is failure to clearly identify the true over-the-back position on the femur in the posterior aspect of the notch. A relative prominence or change in slope is routinely present 3 quarters of the way back on the roof to lateral border of the notch, which can easily be mistaken for the true-over-the back position. William G. Clancy, Jr., M.D., originally coined the term “resident’s ridge” (Clancy, personal communication, 1998). “. . . when residents were beginning to learn how to do our anterior cruciate ligament reconstruction, they did not adequately debride the entire
Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 19, No 9 (November), 2003: pp 931-935
931
932
M. R. HUTCHINSON AND S. A. ASH
posterior aspect of the lateral femoral condyle and assumed they were looking at the most posterior edge when they saw the ‘resident’s ridge.’” The resident’s ridge is a vertical to transverse ridge approximately 75% posterior from the anterior aspect of the femur on the lateral femoral condyle with the knee flexed to 90° (Fig 1). Mistaking resident’s ridge for the true overthe-back position leads to anterior tunnel placement and, therefore, premature failure of reconstructions. When performing arthroscopic notchplasties, we noticed that resident’s ridge appeared to be harder than the bone of the lateral intercondylar notch anterior to it. This change frequently caused the arthroscopic burr to jump or skip. We hypothesized that the exact anatomy that accounted for resident’s ridge may be because of a cortical thickening that served as a bumper to the anterior cruciate ligament. This would explain both the ridge and our arthroscopic findings when burring the ridge. The purpose of this study was, therefore, to characterize the anatomy of the lateral wall and roof of the intercondylar notch, to better define resident’s ridge, and provide an explanation for the arthroscopic findings during burring of this area. Increased awareness and knowledge should, in turn, allow the surgeon to make a reproducible femoral tunnel for ACL reconstruction. METHODS Ten human distal femurs were harvested from embalmed cadavers at the completion of their use in the University of Illinois at Chicago medical school anatomy laboratory. None had been previously dissected in the area of interest. The specimens were dehydrated in ethyl alcohol and cleared in xylene. They were infiltrated with 3 different viscosities of methylmethacrylate and then polymerized in methyl methacrylate. Cross-sections 2-mm thick were made using an Exakt cutting system diamond band-saw blade, 0.3-mm thick (Exakt Technologies, Oklahoma City, OK). The plane of the cross-sections was a slightly canted sagittal section selected by approximating the 11- or 1-o’clock position for the right or left femur, respectively, and then using a plane parallel to this. The cut specimens were surface stained with Paragon stain to distinguish cartilage and soft tissues from bone. The canted sagittal cross-sections were cut from medial to lateral based on the width of the ACL attachment. The cross-section judged to be in the center of the ACL attachment from medial to lateral was selected for measurement of the cortical thickness at the center of the ACL attachment, the anterior
FIGURE 1. (A) Arthroscopic view of right knee identifying resident’s ridge with a probe. Note the true over-the-back position posterior to the ridge. (B) Arthroscopic view of anterior placement of the femoral tunnel if resident’s ridge is used instead of the true over-the-back position. (C) Arthroscopic view of femoral guide in appropriate placement. Note the presence of a remnant of resident’s ridge and the old ACL stump.
condylar hyaline cartilage junction, a point midway between the ACL and the condylar cartilage junction, and resident’s ridge (usually noted 1 mm anterior to the anterior edge of the ACL attachment). This cross-
RESIDENT’S RIDGE
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TABLE 1. Cortical Thickness at Various Sites on the Roof of the Intracondylar Notch Specimen
Cartilage Junction
Midway Point
Resident’s Ridge
ACL Insertion
1 2 3 4 5 6 7 8 9 10 Mean SD
1.26 0.50 1.46 1.32 1.00 1.00 0.76 0.50 1.08 0.76 0.96 mm 0.31 mm
1.38 0.32 1.12 1.62 1.12 0.88 0.62 0.96 0.62 0.36 0.90 mm 0.40 mm
1.62 0.30 1.88 0.28 1.70 0.92 0.46 0.56 1.00 0.26 0.90 mm 0.60 mm
1.76 1.40 2.62 1.76 1.56 2.26 0.70 1.50 1.48 0.96 1.60 mm 0.51 mm
(Model No. 1048; American Optical, Southbridge, MA) with Olympus eyepieces (Olympus, Melville, NY) equipped with a calibrated measuring scale was used to measure the cortical thickness. Measurements were made to the nearest 0.02 mm. A randomized block design with the Tukey adjustment for multiple comparisons was used to analyze differences in site thickness (SAS/STAT v. 6.12 software for SAS Institute, Cary, NC). RESULTS
FIGURE 2. (A) Canted cross-sectional sagittal section of distal femur specimen with (B) accompanying line drawing. The solid stout arrow points to the change in slope of the notch roof that correlates with resident’s ridge. The acute curved arrow points to the hyaline cartilage on the anterior aspect of the femur in the trochlea. The 2 hollow arrows point to the cortical thickening beneath the attachment of the anterior cruciate ligament. The solid curved arrow references the insertion and fibers of the anterior cruciate ligament. The 3 triangular arrows point to the posterior capsule and its attachment on the posterior aspect of the femur.
section was also examined qualitatively for the presence or absence of a ridge anterior to the ACL attachment and the change of slope noted there (Fig 2). Any raised ridge of greater than 1 mm or change in slope of greater than 5° was considered consistent with the presence of a ridge. An American Optical microscope
The mean thickness of cortical bone was 0.96 mm at the cartilage-notch junction site, 0.90 mm at the midway point, 0.90 mm at resident’s ridge, and 1.60 mm at the ACL attachment site (Table 1). Multiple comparison testing between sites was performed. The only sites with a significant difference were related to the ACL attachment (Table 2). Although the standard deviation was relatively broad, specimen 5 was the only specimen in which any measurement (in this case resident’s ridge) exceeded the ACL attachment measurement. TABLE 2. Statistical Comparisons Between Sites Comparison
P
Cartilage junction v midway point Cartilage junction v resident’s ridge Cartilage junction v ACL insertion Midway point v ACL insertion Midway point v resident’s ridge Resident’s ridge v ACL insertion
.9790 .9735 .0016* .0006* 1.000 .0005*
*Statistically significant by randomized block design with Tukey adjustment for multiple comparisions.
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M. R. HUTCHINSON AND S. A. ASH
Observations regarding the contours of the crosssections near the anterior border of the ACL attachment were instructive. We defined a significant change in slope as greater than 5°. In 9 of 10 specimens, the lateral wall cortex appeared to change slope just anterior to the thickening of the ACL attachment site. This change in slope corresponded to what is commonly referred to as resident’s ridge. DISCUSSION Although some studies have documented the dimensions of the notch and femoral attachment site, none has evaluated the cortical thickness of the lateral wall and roof of the notch or the bony anatomy accounting for resident’s ridge.1-8 Surgical awareness of notch roof anatomy is important.9-10 A surgeon who inadvertently mistakes resident’s ridge for the true over-the-top position at the back of the intercondylar notch will create an unacceptably anterior femoral tunnel. Carson et al.10 noted that the most frequent surgical error causing ACL reconstruction failure is anterior femoral tunnel placement. Our findings document the anatomy of resident’s ridge and hopefully will assist surgeons in its recognition to avoid malposition of the femoral tunnel placement. During preparation of the intercondylar notch, many surgeons will use a burr to take down or flatten resident’s ridge to assure clear visualization and access to the over-the-top position. During this notch preparation, the burr will frequently jump at the apparently harder or thicker bone of resident’s ridge. Theoretically, this finding could be secondary to a number of factors. These factors include that resident’s ridge could have a thicker cortex and denser bone that would serve as an anterior bumper for the ACL (our original hypothesis); that the entire femoral attachment of the ACL could have a thicker cortex, with resident’s ridge representing only the leading edge of the more prominent cortex and dense bone; or that clinical observation of denser bone could be inaccurate and that no variation in the cortical thickness exists relative to the intercondylar notch, resident’s ridge, and the femoral ACL attachment. Our findings support the second of these possibilities. The femoral attachment of the ACL is the only region we measured with a significantly thicker cortex. It is thicker than resident’s ridge, the hyaline cartilage-intracondylar notch junction, and the point midway between the ACL attachment and the hyaline cartilage-intracondylar notch junction. Resident’s ridge, in and of itself, does not have a thicker cortex.
Variable cortical thickness is a challenge the surgeon must contend with to avoid erratic burr behavior. In addition, the change in slope evident in 90% of our specimens probably was a contributing factor. Potential limitations of this study include the selection of specimens and the relative broad standard deviation noted regarding cortical thickness measurements at each site. The variability of cortical thickness between specimens is probably tied to the relatively broad range of ages of the embalmed specimens chosen. One would reasonably expect that the younger specimens would reveal thicker cortices and denser bone; nonetheless, that the relationship of cortical thickness between sites should be significantly altered is unlikely. Osteopenia and osteoporosis are, in general, diffuse processes and should affect each site at relatively the same pace. More specifically, regardless of age, the relative increased cortical thickness at the ACL attachment site should be thicker than the resident’s ridge, the hyaline cartilage-intracondylar notch junction, and the point midway between the ACL attachment and the hyaline cartilage intracondylar notch junction. Indeed, despite the broad range of standard deviation of measurements between specimens, only 1 specimen (specimen No. 5) had any measurement that exceeded the cortical thickness measured at the ACL attachment. In conclusion, resident’s ridge is a change in slope or ridge located about 75% posteriorly on the roof and lateral wall of the intracondylar notch of the knee. It is located just anterior to the ACL attachment and anterior to the posterior limit of the intracondylar notch (the over-the-top position). Resident’s ridge is a consistent anatomic landmark identifiable in 9 of 10 knees we studied. Resident’s ridge represents the change in slope of the notch roof as well as a transitional thickness of cortex from notch to actual ACL attachment point. The term resident’s ridge was coined by William Clancy, Jr., M.D. The significance of resident’s ridge is that it can be misinterpreted as the posterior roof of the notch and can inadvertently lead to nonanatomic anterior placement of the femoral tunnel for ACL reconstruction. Special care when creating the femoral tunnel for ACL reconstruction should include an awareness of resident’s ridge. Carefully reducing the ridge with a burr will assist in confirming the true over-the-top position as the appropriate anatomic landmark. This should make accurate tunnel placement more reproducible. Finally, variable cortical thickness is a challenge the surgeon must face to avoid erratic burr behavior. The variation in cortical density
RESIDENT’S RIDGE is explained by the femoral attachment of the ACL itself and not the resident’s ridge. Acknowledgment: The authors thank Susan King, H.T. (A.S.C.P.), Robert Anderson, Ph.D., Luna Ghosh, M.D., Karen Ferrer, M.D., and Taran Bae, M.D., for their technical assistance.
REFERENCES 1. Anderson AF, Lipscomb AB, Liudahl KJ, et al. Analysis of the intercondylar notch by computed tomography. Am J Sports Med 1987;15:547-552. 2. Herzog RJ, Silliman JF, Hutton K, et al. Measurements of the intercondylar notch by plain film radiography and magnetic resonance imaging. Am J Sports Med 1994;22:204-210. 3. Muneta T, Takakuda K, Yamamoto H. Intercondylar notch width and its relation to the configuration and cross-sectional area of the anterior cruciate ligament. Am J Sports Med 1997; 25:60-72.
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4. Good L, Odensten M, Gillquist J. Intercondylar notch measurements with special reference to anterior cruciate ligament surgery. Clin Orthop 1991;263:185-189. 5. Houseworth SW, Mauro VJ, Mellon BA. The intercondylar notch in acute tears of the anterior cruciate ligament: A computer graphics study. Am J Sports Med 1987;15:221-224. 6. Mensch JS, Amstutz HC. Knee morphology as a guide to knee replacement. Clin Orthop 1975;112:231-241. 7. Shelbourne KD, Davis TJ, Klootwyk TE. The relationship between intercondylar notch width of the femur and the incidence of anterior cruciate ligament tears: A prospective study. Am J Sports Med 1998;26:402-408. 8. Girgis FG, Marshall JL, Al Mohajem ARS. The cruciate ligaments of the knee joint: Anatomical, functional, and experimental analysis. Clin Orthop 1975;106:216-231. 9. Dalton JD, Harner CD. Surgical techniques to correct nonanatomic femoral tunnels. Oper Tech Sports Med 1998;6:8390. 10. Carson EW, Simonian PT, Wickiewicz TL, et al. Revision anterior cruciate ligament reconstruction. Instr Course Lect 1998;47:361-368.