Primary Hip Arthroplasty Case Title: Uncemented Primary Total Hip Arthroplasty (Using an Extensively Porous-Coated [Anatomic Medullary Locking, AML] Stem)
Case Title: Uncemented Primary Total Hip Arthroplasty (Using
an Extensively Porous-Coated [Anatomic Medullary Locking, AML] Stem)
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Demographics
Age: 61 Sex: Female BMI: 34.1
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Relevant Past Medical History
Principal pathologies: Hypertension, hyperlipi-daemia and coronary artery disease with previous myocardial infarction.
Previous surgical procedures: Tonsillectomy and adenoidectomy.
Medication: Lisinopril 10 mg daily and Crestor 4 mg nightly.
History of presenting complaint: Our patient was a 61-year-old woman with right hip pain for the past year, predominately located in the groin precipitated by walking. She has difficulty with daily activities such as putting on shoes and socks. She has tried Tylenol with some relief, has received an intra-articular hip injection which only lasted one day and has also performed physical therapy with limited benefit.
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Clinical Examination
Symptoms: Right groin pain with walking and daily activities.
Range of motion: Extension: 0°; flexion: 70°; abduction: 20°; adduction: 10°; external rotation:
20°; and internal rotation: 10°.
Specific tests: Antalgic gait.
Main disability: Ability to ambulate.
Neurovascular evaluation: 5/5 DF, PF, EHL and FHL with 2+ DP/PT pulses.
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Preoperative Radiological
Assessment/Imaging (Figs. 3.14,
3.15, and 3.16)
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Fig. 3.14 Preoperative AP radiograph demonstrating severe osteoarthritis of the right hip with significant loss of joint space, osteophyte formation and subchondral sclerosis
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Fig. 3.15 Preoperative lateral radiograph demonstrating the same as above
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Fig. 3.16 Preoperative AP radiograph with template imaging overlaid. Preoperatively we templated a standard offset femoral stem with a 15 mm distal diameter and an acetabular cup with a 52 mm diameter. The patient was also noted to have a 4.7 mm leg length discrepancy
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Preoperative Planning
Diagnosis: Osteoarthritis of the right hip (Figs. 3.14 and 3.15).
Possible treatment options: Nonoperative, however the patient already has exhausted all non-surgical treatment options to include activity modification, physical therapy, oral pain medications and intra-articular injections, and her pain has continued to affect her basic activities of daily living.
Chosen treatment method: Primary cementless total hip arthroplasty with an extensively porous-coated femoral stem featuring a cylindrical distal geometry.
Selection of implants if applicable and rational: At the time of the surgery, our institution predominately used an uncemented extensively porous-coated stem for femoral fixation as they achieve immediate diaphyseal cortical fixation and have been shown to have 98% survivorship at 20 years. A fully porous-coated implant allows for the greatest surface area for cortical bone-implant apposition and the best potential for ingrowth into the prosthesis.
Expected difficulties: Due to the importance of achieving adequate distal scratch fit between the
prosthesis and the femur, it is imperative that the direction of the femoral reamer is controlled by the diaphysis and not by more proximal structures such as the greater trochanter or femoral neck. To the surgeon less experienced with implanting a fully porous-coated stem, this may be easily overlooked and risks malpositioning or undersizing the component.
Strategies to overcome difficulties: To help avoid a malpositioned or undersized component, in addition to an adequate exposure and soft tissue releases, it may be helpful to use a high-speed burr to create the starting pilot hole for femoral reaming. This helps to remove potential proximal impingement structures such as a portion of the overhanging greater trochanter.
Templating: The leg lengths were measured on the preoperative AP radiograph, and the right lower extremity was found to be 4.7 mm shorter (Fig. 3.16). The next step in templating is to identify the correct location of the true acetabulum, the correct size and the inclination and evaluate the extent of bony contact between the implant and host bone. We templated to have the inferior border of the cup at the inferior aspect of the tear-drop and the medial aspect of the cup adjacent to the medial wall of the acetabulum with approximately 45 degrees inclination. With the chosen location and size of the cup, we felt comfortable that there was adequate bony contact between the host bone and the superior lateral aspect of the cup implant. With the cup centre established, the location of the femoral neck cut and size and offset of the femoral implant are determined. The location of the neck cut is much more critical for a cementless stem compared to a cemented stem because cutting the neck too long or too short will not allow the implant to sit properly within the medullary cavity and result in unintentionally lengthening or shortening of the limb. With this implant, we template to have the lateral aspect of the stem approximately 12 mm below the tip of the greater trochanter. In this case, we templated a femoral stem with a standard offset neck and a 15 mm distal diameter that appeared to fill the diaphyseal canal, restore the native offset and allow for at least 5 cm of distal scratch fit.
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Surgical Note
Patient’s position: Lateral decubitus position with the left hip down.
Type of anaesthesia: Spinal anaesthesia with conscious sedation.
Surgical approach: Posterior approach with posterior capsule and external rotator repair.
Main steps: For implantation of a fully porous-coated cementless prosthesis, any surgical approach may be utilized. However, the direct anterior approach is difficult with a long straight stem such as this. In the lateral position for a posterior approach, the surgeon must ensure the pelvis is tightly secured to the table and perpendicular to the floor to allow for easier intraoperative determination of leg lengths and acetabular version. After the surgical approach and adequate visualization of the head/neck and acetabulum, the femoral neck osteotomy is made at the level of the templated cut. Once the femoral osteotomy has been made, attention is turned to preparing and implanting the acetabulum. However, for the purposes of this chapter, we will continue discussing the femoral side of the arthroplasty. The first step in preparing the femoral canal is locating the correct starting point for canal reaming. It is imperative that the pilot hole is in the correct location, which is just slightly anterior to the piriformis fossa and right off the medial wall of the greater trochanter, in order to prevent reamer deflection off the trochanter resulting in a varus stem position. Start with a small size reamer and ream incrementally up by 1 mm until cortical contact is encountered, and at which time, sequentially increase the size of the reamer by
0.5 mm. Continue to ream until a cutting resis-
tance in the proximal femoral diaphysis is encountered for approximately 5–7 cm. There should be visible cortical bone between the flutes of the last reamer. The final reamer should be
0.5 mm smaller than the implant size in order to achieve maximal diaphyseal scratch fit. The final step in canal preparation is incremental broaching of the proximal femur to accept the triangular geometry of the AML stem. Once the final broach size is achieved and proper stability is confirmed, trialling may proceed as indicated by the leg lengths and offset (Figs. 3.17 and 3.18).
Reconstruction techniques: Reconstruction using a fully porous-coated implant follows the general principles of primary total joint arthroplasty for osteoarthritis, and the specific steps for achieving proper component size and position are described elsewhere in this chapter.
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Intraoperative Challenges
Challenges and solutions: Intraoperative femoral fractures during stem insertion can occur with cylindrical, extensively porous-coated stems. These fractures can be proximal, around the stem or distal to the stem tip. Proximal femur fractures typically occur as a result of a geometric mismatch between the triangular pattern of the implant and the femoral metaphysis. Risk factors that may predispose one to this are patients of smaller stature, female gender, a low femoral neck cut and femoral coxa vara. If there is a sudden ‘jump’ in stem position during impaction or the stem begins to advance more rapidly with successive blows, this should alert the surgeon to the possibility of a fracture that can be evaluated with intraoperative X-ray. A non-displaced distal femur fracture noted intraoperatively can be treated using a cerclage wire with protected weight-bearing postoperatively. In the rare event of a distal displaced fracture, the surgeon must be prepared to extend the approach distally, expose the fracture site and perform standard internal fixation principles with plates, screws and cables. Fractures noted on postoperative radiographs that are distal to the stem tip and non-displaced can be treated with or protected from weight-bearing and typically heal uneventfully. These hairline fractures are only apparent on either the AP or lateral radiograph just distal to the tip of the stem.
Unanticipated problems and solutions: Femur
insertional fractures are an unanticipated problem but have been reported to occur approximately 3% of the time when using an AML-type prosthesis.
Thorough description of decision-making, including the reason for the final decision: Extensively porous-coated femoral stems, were pioneered at our institution. Because of the
ability to achieve immediate distal cortical fixation with a reproducible surgical technique, these stems became our implant of choice for primary total hip arthroplasty. The long-term durability of the fixation has also been demonstrated with a stem survivorship rate of 97.8% at twenty years using revision for any reason as an endpoint.
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Postoperative Radiographs
(Figs. 3.17 and 3.18)
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Fig. 3.17 Immediate postoperative AP radiograph demonstrating adequate sizing of the femoral component with 5 cm of canal filling, diaphyseal cortical fit, restoration of leg lengths and appropriate cup orientation
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Fig. 3.18 1-year postoperative AP radiograph demonstrating stable component position without evidence of osteolysis or loosening and adequate distal stem osseous ingrowth
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Postoperative Management
Chemoprophylaxis and anticoagulant treatment period: For deep venous thrombosis prophylaxis, the patient was started on low-dose warfarin therapy with a goal INR of 1.5–2 for a period of thirty days. Gait/limb loading until full loading: The patient was allowed 50% weight-bearing on the right lower extremity for three weeks postoperatively and then
transitioned to full weight-bearing as tolerated.
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Follow-Up and Complications
There were no complications related to the management of this patient.
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Discussion [36–39]
Advantages of the applied method: The two main advantages of performing an uncemented primary hip arthroplasty with an extensively porous-coated stem such as the AML are the cementless design and immediate distal cortical fixation. Cementless arthroplasty was popularized in the 1980s, and although there is no consensus about the optimal technique for femoral fixation, cementless stems were preferred in younger patients with adequate bone stock without abnormal morphologic features of the femur. Twenty-year results of this stem have demonstrated proven success across a spectrum of diseases. Although other designs of cementless stems have 20-year results, the AML was one of the first successful cementless stems implanted. Successful cementless fixation requires rigid interface fit to the host bone strong enough to support body weight. By reaming until cortical chatter is encountered, and then press fitting a 0.5 mm larger stem into the canal, the surgeon achieves instant distal cortical fixation that is stable for immediate postoperative weight-bearing. Retrieval studies have demonstrated intimate cortical ingrowth into these stems that can be seen on postoperative radiographs as thick, dense, hypertrophied cortices distally around the implant.
Disadvantages of the method: The same fea-
tures that contribute to the advantages of exten-
sively porous-coated cylindrical stems like the AML are also associated with some potential drawbacks. The distal cortical bony ingrowth and the transfer of stresses that occurs from the implant to the bone at this point can result in proximal stress shielding and bone density loss in the intertrochanteric region. While proponents for proximally coated stems contend that stress shielding results in greater incidences of thigh pain, the potential for fracture of the femur, loss of implant fixation and increase in the severity of osteolysis and can cause difficulties during revision, there is currently no literature that definitively substantiates these theoretical disadvantages. Countering these concerns, long-term follow-up data has shown durable fixation for these implants with ~98% survivorship at twenty years, and studies have shown no correlation between the degree of radiographic stress shielding and incidence of thigh pain. Bone mineral density studies have also shown no correlation between hip and pain scores and bone density changes. Additionally, retrieval analysis studies have shown no correlation between stress shielding and radiographic evidence of osteolysis or clinical evidence of stem loosening. Due to the extensive cortical ingrowth that occurs with this type of stem, in the event of a revision surgery where the stem needs to be removed, an extensile approach such as an extended trochanteric osteotomy is typically necessary in order to safely remove the stem that has a cortical bone grown into it. A more detailed discussion of these revision approaches is discussed elsewhere.
Alternative evidence-based techniques for the
case: For cementless total hip arthroplasty, the other more commonly used implant is a proximally porous-coated stem. Proponents of these implants cite a decreased incidence of postoperative thigh pain and proximal stress shielding compared to extensively porous-coated implants. Contraindications to using a proximally porous-coated stem are few if any.
Why is the chosen technique better for this case? As discussed previously, we believe that the extensively porous-coated stem allows for immediate distal cortical fixation, and complications such as thigh pain can be limited by following our
steps to ensure proper component positioning and sizing.
Indications and contraindications for your technique: Indications for the use of this extensively porous-coated cementless stem include primary total hip arthroplasty for osteoarthritis, rheumatoid arthritis, avascular necrosis of the femoral head, post-traumatic arthritis and intra-capsular femoral neck fractures. These stems may also be utilized in revision surgery for treatment of a periprosthetic fracture with adequate proximal bone stock or revision for reasons other than fracture where the patient has a proximal bone loss that would not support a proximally porous-coated press-fit stem. The only absolute contraindication for the use of this stem, or any cementless stem for that matter, is an active infection. Relative contraindications include patients with femoral diameters of less than 10 mm or larger than 21 mm or patients with significant preoperative femoral anteversion, such as the case in a hip dysplasia patient, where retroverting the stem to a normal version would not allow for adequate canal filling distally. In this instance, the surgeon may want to consider a cemented or a modular design stem.
Learning curve and how to manage compli-
cations: As previously discussed, the incidence of thigh pain following implantation of extensively porous-coated stems has been shown to be 3–8%. Controversy exists regarding the true aetiology of thigh pain, as historically it has been associated with a loose femoral component. An inexperienced surgeon implanting this type of stem may encounter the initial cortical chatter and presume that this is the final size for the stem, inadvertently undersizing the component. In actuality, once the initial chatter is heard, the surgeon must continue incrementally reaming 0.5 mm size at a time until cortical bone is noticed between the reamer flutes on the final one or two reamers. Once the surgeon believes that the last reamer has been used, a method to check and determine the amount of distal scratch fit present is to use the same size reamer as the final femoral stem and insert it until it comes into contact with the diaphysis.
The proximally exposed portion of the reamer estimates the distance of the diaphyseal cortical contact. It is important to note that proximal bone loss secondary to stress shielding is not a complication, but rather a sign of success. Only bone ingrown implants demonstrate stress shielding related to bone loss.
Level of evidence concerning the superiority of this method against others: The highest level of evidence reporting on the use of these extensively porous-coated femoral stems is level IV.
