The Ceramic Option: Indications, Contraindications, Revision and Surgical Cinallenges 133
Acetabular Positioning without Navigation Anterior Approacli
W. J. Hozack
Introduction
Navigated preparation of acetabular bone and placement of acetabular components is a new tool available to the orthopedic surgeon - traditional techniques are still the most commonly used. The goal of this chapter is to highlight those traditional techniques vs/hich can ensure reliable component positioning and con minimize complications such as instability and leg length discrepancy which occur when acetabular component position is not ideal.
Successful placement of an acetabular component can be divided conveniently into three steps:
1. exposure
2. bone preparation 3. component insertion
Exposure
Regardless of the surgical approach chosen, the quality of the surgical exposure significantly influences the quality of the clinical result. The critical facet of each hip exposure is identification of all key bony landmarks. This requires appropriate removal of both soft tissue and bone tissue.
Soft Tissue Removal
Soft tissue impediments to proper acetabular exposure are the labrum and the foveal contents. The acetabular labrum overhangs the bony margins of the acetabulum (especially in acetabular dysplasia) and must be excised in its entirety prior to reaming of the socket. Failure to do so can lead to over-reaming of either the anterior or posterior column or both. Occasionally the labrum is fully calcified and must be removed with a rongeur. Foveal contents are invariably present even in the most deformed acetabulum (with the exception of protrusio cases), and need to be identified and removed prior to proceeding with any acetabular preparation. These foveal contents allow the surgeon to identify the teardrop, which locates the inferior position of the acetabular component.
Bony Landmarks
Acetabular bony landmarks that need to be identified and are critical to the surgical result are: anterior column, posterior column, ilium, and teardrop. The acetabular teardrop represents the anatomic position of the acetabulum. Cups placed inferior to the teardrop can create leg length discrepancies. Full identification of the anterior and posterior columns prevents improper reaming location and ensures proper cup location. Full identification of the ilium prevents inadvertent lateralization of the hip center of rotation. In both the regular and especially in the most deformed acetabulum, the easiest and safest way to
I 34 SESSION 4.3
identify the b o n y landmarks is to work your w a y d o w n fronn the ilium across the anterior a n d posterior columns to identify the pubis, ischium, a n d t e a r d r o p .
Bone Removal
Bony impediments to a c e t a b u l a r exposure are l o c a t e d in the foveal area a n d also peripherally a b o u t the a c e t a b u l a r rim. Evaluation of the preoperative radiographs c a n provide some g u i d a n c e as to the presence of these potential problems. The foveal area c a n b e completely obscured with osteophytes, thus preventing proper identification of the a n a t o m i c position of the a c e t a b u l u m . These o v e r h a n g i n g osteophytes must b e excised completely. Peripheral rim osteophytes c a n confuse the surgeon as to the proper a c e t a b u l a r a n a t o m y . Lateral osteophytes d e v e l o p as a c o n s e q u e n c e of certain types of arthritis - reliance o n these osteophytes for m e c h a n i c a l support of the a c e t a b u l a r c o m p o n e n t is a recipe for failure. Further, certain osteophytes c a n direct a reamer in a n inappropriate direction with potentially disastrous consequences. Failure to r e m o v e large inferior osteophytes c a n push reaming proximally from the a n a t o m i c position.
Bone Preparation
No a t t e m p t s to p r e p a r e the b o n e of the a c e t a b u l u m should b e undertaken until all a n a t o m i c landmarks of the a c e t a b u l a r a n a t o m y h a v e b e e n identified properly.
First Reamer
A c e t a b u l a r r e a m i n g should follow specific guidelines. The first a c e t a b u l a r reamer should b e significantly smaller t h a n the true a n a t o m i c size - this m a y require extremely small reamers (36 - 39 mm) in certain situations. The initial reamer is used to medialize subsequent reamers to the a n a t o m i c position.
Subsequent Reamers
Subsequent r e a m i n g of t h e a c e t a b u l u m requires a three dimensional a p p r o a c h . Ultimately the a c e t a b u l a r c o m p o n e n t must b e p l a c e d concentrically within the b o n y confines available. My a p p r o a c h is to visualize the ultimate position of the a c e t a b u l a r c o m p o n e n t within the a c e t a b u l a r b e d , a n d then the gradually fit the reamers to a c h i e v e this g o a l . A n a t o m i c guides are the teardrop off w h i c h the c u p hinges, a n d the anterior a n d posterior columns that ultimately d e t e r m i n e the size of the a c e t a b u l a r c o m p o n e n t . The w e a r a n d tear of osteoarthritis often creates a situation in w h i c h reaming achieves a n anterior- posterior fit prior to full c o n t a c t of the reamer with the superior rim. The surgeon should not feel o b l i g a t e d to continue reaming until full c o n t a c t is o b t a i n e d circumferentially - to d o so w o u l d critically compromise the b o n y integrity of the a c e t a b u l u m . As long as a pressfit c a n b e a c h i e v e d b e t w e e n the anterior a n d posterior columns, partial lack of c o v e r a g e of t h e c u p laterally will not c o m p r o m i s e c o m p o n e n t fixation.
C o m p o n e n t orientation is s o m e w h a t d e p e n d e n t u p o n surgical a p p r o a c h . A b d u c t i o n a n g l e of the socket is 30 to 45 degrees - this a n g l e should b e c r e a t e d during r e a m i n g . Anteversion varies s o m e w h a t . The a n a t o m i c anteverslon of the a c e t a b u l u m ( o n c e osteophytes are r e m o v e d ) is 10 - 15 degrees. This is
The Ceramic Option: Indications, Contraindications, Revision and Surgical Challenges 135
acceptable for anterior approaches, but posterior approaches may require 20 degrees in order to avoid higher dislocation rates.
Component Insertion
Complicated external jigs or guides for acetabular component insertion ore undependable - the surgeon must rely upon anatomic clues. As mentioned previously, proper identification of the bony landmarks is essential. Off these anatomic landmarks are based all decisions.
Specific Issues Related to the Anterior Approach
The experience with total hip replacement using the anterior approach at Rothman Institute Orthopedics has evolved over 30 years. During this period, we have found that performing the surgery with the patient in the supine position has offered us tremendous advantage in terms of hip stability and leg length equality.
It is my opinion that the supine patient position allows the surgeon to "navigate"
the position of the pelvis at all times during the surgical procedure. Very much like current computer assisted navigation, the surgeon can easily palpate the anterior superior iliac spines and the pubic symphysis - something not easily done when the patient is in the lateral decubitus position. This allows the surgeon to utilize his brain as a computer (infinitely more sophisticated than any computer program) to properly orient himself when reaming the acetabulum and when inserting the acetabular component.
