Orthopedics

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Acetabular Cup Position: The Imperative of Getting It Right

Lawrence D. Dorr, MD

Abstract

The importance of acetabular cup position is best defined by the avoidance of impingement. Impingement is either stem-on-cup or bone-on-bone. Stem-on-cup is caused by a too-small head-neck ratio and incorrect mating of the cup and stem position. Bone-on-bone impingement is caused by incorrect reconstruction of the hip length and offset. Impingement causes dislocation, accelerated wear, and pain.1

The clinical occurrence of impingement is not known because it is difficult to show impingement. It is not evident on radiographs. Implant retrieval has confirmed that it occurs. The study of Shon et al2 correlated impingement with the complication of dislocation. Yamaguchi et al3 demonstrated that the most common site of impingement is at 10 o’clock on the cup surface. This can occur easily if there is insufficient anteversion of the femoral stem or if the combined anteversion is too low. It also will occur if the acetabular cup is lateralized. It occurs most commonly with either abduction of the hip or external rotation and extension of the leg as in gait.

The acetabular cup position should not be described in isolation when discussing complications of the hip. The hip joint in any given position (sitting, standing, or supine) is a combination of the acetabular and femoral anteversion (combined anteversion). Pierchon et al4 have shown that the cause of dislocation cannot be determined by measuring cup anteversion alone. Komeno et al5 have shown that the cause of dislocation correlates to combined anteversion. Combined anteversion has a safe zone of 25° to 45°, with 25° to 35° for men and 30° to 45° for women.6,7 Our mean combined anteversion in our clinical series is 37°, and 37° is the recommended combined anteversion in the finite element study of Widmer and Zurfluh.8

If femoral anteversion can be controlled at 15°, as with cemented stems, the cup can be targeted to 15° to 25° depending on gender. Therefore, when cemented stems were prevalent, the focus was a safe zone for the cup such as 15°+10° or 20°+10°.9,10 Combined anteversion has become more relevant since the cementless femoral stem is essentially inflexible. The cup position cannot be targeted and must be adapted to stem anteversion. Therefore, at surgery, the femur is prepared first and the stem anteversion determined, and the cup is then anteverted according to the stem anteversion to provide a combined anteversion within the safe zone.

The femoral stem anteversion can be estimated by the surgeon within 10° most often after 15 to 20 consecutive estimates as a learning curve. It is also easier with a trial neck on the broach or with a trial stem to correctly estimate the stem anteversion. The stem anteversion can be learned with experience because the axis of the leg can be visualized (Figure 1). The surgeon will have 5% to 10% outliers between 5° and 10° because the bow of the femur cannot be visualized (Figure 2). The femoral stem anteversion is controlled by the anteversion of the femur, the anteroposterior isthmus of the femur at the level of the lesser trochanter (which creates 3-point fixation), and the bow of the diaphysis, which is also influenced by the thickness of the posterior fin of bone, such as in Dorr type A and B.11

Surgeon estimation of cup anteversion has 10% to 15% outliers beyond 10°.12 This occurs because the pelvic tilt cannot be visualized. Pelvic tilt is ≥10° in 16% of hips, so the correlation of tilt to surgeon outliers is close. The surgeon is close with judgment of the cup anteversion in 85% to 90% of hips, but can be wrong by ≥10° in 10% to 15%.…

Lawrence D. Dorr, MD
Lawrence D. Dorr
 

The importance of acetabular cup position is best defined by the avoidance of impingement. Impingement is either stem-on-cup or bone-on-bone. Stem-on-cup is caused by a too-small head-neck ratio and incorrect mating of the cup and stem position. Bone-on-bone impingement is caused by incorrect reconstruction of the hip length and offset. Impingement causes dislocation, accelerated wear, and pain.1

The clinical occurrence of impingement is not known because it is difficult to show impingement. It is not evident on radiographs. Implant retrieval has confirmed that it occurs. The study of Shon et al2 correlated impingement with the complication of dislocation. Yamaguchi et al3 demonstrated that the most common site of impingement is at 10 o’clock on the cup surface. This can occur easily if there is insufficient anteversion of the femoral stem or if the combined anteversion is too low. It also will occur if the acetabular cup is lateralized. It occurs most commonly with either abduction of the hip or external rotation and extension of the leg as in gait.

 
Figure 1: The correct position for judging the anteversion of the femoral component
Figure 1: The correct position for judging the anteversion of the femoral component. The assistant should put a finger on the epicondyle so the surgeon knows its position. The anteversion of the stem is then sighted along the posterior thigh in reference to the epicondyles. Do not use the tibia because there may be deformities in the knee or in the tibia. Using the epicondyle with computer navigation gives a precision of 4.87. The surgeon can get to be proficient at estimating the stem anteversion within 57 with a learning curve of 15 to 20 stems. Figure 2: The AP plane of the femur (lateral radiograph)
Figure 2: The AP plane of the femur (lateral radiograph). A tapered stem is stabilized in the femoral canal by the posterior neck, in the anterior edge of the isthmus at the level of the lesser trochanter, and in the diaphysis by the posterior fin with the tip commonly against the anterior cortex. With a narrow isthmus or a large diaphyseal bow the stem is forced into retroversion. With more osteoporotic bone (Dorr type C) the stem can be inserted with the anatomic anteversion into the canal because there is no posterior fin. The anteversion of the stem is therefore controlled by the anteversion of the femoral neck and the anteroposterior isthmus in this bone.

The acetabular cup position should not be described in isolation when discussing complications of the hip. The hip joint in any given position (sitting, standing, or supine) is a combination of the acetabular and femoral anteversion (combined anteversion). Pierchon et al4 have shown that the cause of dislocation cannot be determined by measuring cup anteversion alone. Komeno et al5 have shown that the cause of dislocation correlates to combined anteversion. Combined anteversion has a safe zone of 25° to 45°, with 25° to 35° for men and 30° to 45° for women.6,7 Our mean combined anteversion in our clinical series is 37°, and 37° is the recommended combined anteversion in the finite element study of Widmer and Zurfluh.8

If femoral anteversion can be controlled at 15°, as with cemented stems, the cup can be targeted to 15° to 25° depending on gender. Therefore, when cemented stems were prevalent, the focus was a safe zone for the cup such as 15°+10° or 20°+10°.9,10 Combined anteversion has become more relevant since the cementless femoral stem is essentially inflexible. The cup position cannot be targeted and must be adapted to stem anteversion. Therefore, at surgery, the femur is prepared first and the stem anteversion determined, and the cup is then anteverted according to the stem anteversion to provide a combined anteversion within the safe zone.

The femoral stem anteversion can be estimated by the surgeon within 10° most often after 15 to 20 consecutive estimates as a learning curve. It is also easier with a trial neck on the broach or with a trial stem to correctly estimate the stem anteversion. The stem anteversion can be learned with experience because the axis of the leg can be visualized (Figure 1). The surgeon will have 5% to 10% outliers between 5° and 10° because the bow of the femur cannot be visualized (Figure 2). The femoral stem anteversion is controlled by the anteversion of the femur, the anteroposterior isthmus of the femur at the level of the lesser trochanter (which creates 3-point fixation), and the bow of the diaphysis, which is also influenced by the thickness of the posterior fin of bone, such as in Dorr type A and B.11

Surgeon estimation of cup anteversion has 10% to 15% outliers beyond 10°.12 This occurs because the pelvic tilt cannot be visualized. Pelvic tilt is ≥10° in 16% of hips, so the correlation of tilt to surgeon outliers is close. The surgeon is close with judgment of the cup anteversion in 85% to 90% of hips, but can be wrong by ≥10° in 10% to 15%. This is particularly a danger for impingement if the stem anteversion is also incorrectly estimated by 5° to 10°, which can mean a combined anteversion error of 15° to 25°.

The only technique that a surgeon can use to predictably position the cup 100% of the time is with a “smart tool” such as computer navigation or robotics. In addition, providing a functional cup position requires that imageless computer navigation be used that adjusts cup position to the coronal plane of the body. The precision and accuracy of this technique has been validated.12 Using computer navigation for the cup and stem, the combined anteversion was maintained within the safe zone of 25° to 50° in 96% of hips. Only hips with retroversion of the stem ≥5° have difficulty reaching 25° combined anteversion, and in these hips the stem may need to be cemented or be modular. The inclination of the cup can always be kept below 45° by reaming medially 5 to 6 mm (to the cortical bone of the cotyloid notch) to allow cup coverage with correct inclination.13

At surgery, we prepare the femur first, estimate femoral anteversion, and pack a thrombin-soaked gauze in the canal to minimize bleeding into the acetabulum during its preparation. The acetabulum is then exposed and reamed and a trial cup placed to confirm desired inclination and anteversion with correct coverage. If the cup is prominent anterosuperior and inclination and anteversion are correct, the acetabulum needs to be reamed medially the number of millimeters required for coverage.

The leg length and offset are measured by computer navigation to protect against bone-on-bone impingement. The hip is put through the arc of motion to the extremes of the range of flexion, abduction, adduction/internal rotation, and extension/external rotation to ensure there is no impingement of the lesser trochanter to the ischium or the greater trochanter to the ilium, and the stem to the cup. We use the largest femoral head possible (for the cup size) with highly cross-linked polyethylene for the articulation. The head-cup ratio should be such that the distance between the edge of the head and edge of the cup is <25 mm (32+25 mm=57 mm so the largest cup with 32 mm head is 56 mm).

References

  1. Malik A, Maheshwari A, Dorr LD. Impingement with total hip replacement. J Bone Joint Surg Am. 2007; 89(8):1832-1842.
  2. Shon WY, Baldini T, Peterson MG, Wright TM, Salvati EA. Impingement in total hip arthroplasty a study of retrieved acetabular components. J Arthroplasty. 2005; 20(4):427-435.
  3. Yamaguchi M, Akisue T, Bauer TW, Hashimoto Y. The spatial location of impingement in total hip arthroplasty. J Arthroplasty. 2000; 15(3):305-313.
  4. Pierchon F, Pasquier G, Cotten A, Fontaine C, Clarisse J, Duquennoy A. Causes of dislocation of total hip arthroplasty. CT study of component alignment. J Bone Joint Surg Br. 1994; 76(1):45-48.
  5. Komeno M, Hasegawa M, Sudo A, Uchida A. Computed tomographic evaluation of component position on dislocation after total hip arthroplasty. Orthopedics. 2006; 29(12):1104-1108.
  6. Dorr LD. Hip Arthroplasty: Minimally Invasive Techniques and Computer Navigation. Philadelphia, PA: WB Saunders; 2006.
  7. Ranawat CS, Maynard MJ. Modern techniques of cemented total hip arthroplasty. Tech Orthop. 1991; 6:17-25.
  8. Widmer KH, Zurfluh B. Compliant positioning of total hip components for optimal range of motion. J Orthop Res. 2004; 22(4):815-821.
  9. Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am. 1978; 60(2):217-220.
  10. Barrack RL, Lavernia C, Ries M, Thornberry R, Tozakoglou E. Virtual reality computer animation of the effect of component position and design on stability after total hip arthroplasty. Orthop Clin North Am. 2001; 32(4):569-577, vii.
  11. Dorr LD, Faugere M-C, Mackel AM, Gruen TA, Bognar B, Malluche HH. Structural and cellular assessment of bone quality of proximal femur. Bone. 1993; 14(4): 231-242.
  12. Dorr LD, Malik A, Wan Z, Long WT, Harris M. Precision and bias of imageless computer navigation and surgeon estimates for acetabular component position. Clin Orthop Relat Res. 2007; (465):92-99.
  13. Wan Z, Boutary M, Dorr LD. The influence of acetabular component position on wear in total hip arthroplasty. J Arthroplasty. 2008; 23(1):51-56.

Author

Dr Dorr is from The Arthritis Institute at Good Samaritan Hospital, Los Angeles, California.

Dr Dorr receives royalties from Zimmer.

Correspondence should be addressed to: Lawrence D. Dorr, MD, The Arthritis Institute at Good Samaritan Hospital, 637 S Lucas Ave, 5th Floor, Los Angeles, CA 90017.

10.3928/01477447-20080901-10

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