THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Introduction
Facing increasing life expectancy and numbers of young and active patients undergoing joint replacement, revisions after total hip replacement later in life are to be expected. As a result, the trauma of soft tissues and bone should be kept as minimal as possible in primary surgery.
In order to achieve this aim minimally-invasive approaches have been investigated over the past three decades1 and over the last decade bone and soft tissue preserving implants have been developed.
In particular, preserving the femoral neck has several potential advantages, including increased stability and fixation and improved distribution of stress loads.2 Pipino was the first author to implant neck-preserving anatomically contoured prostheses with a collar.² The Mayo stem, a short-stem prosthesis, employed metaphyseal anchorage with lateral cortical support- designed to provide a high degree of primary stability with proximal anchorage, as well as good antimigration and secondary stability and to facilitate the restoration of physiological joint geometry.3 The femoral neck as a zone for anchorage and load transfer was to be preserved.4 Preservation of bone comes along with preservation of insertions of muscles and tendons responsible for hip posture and movement.
Due to the lack of clinical-data regarding long-term stability for short-stem implants, only biomechanical simulations can indicate that long-term stability might be given.5
Prosthesis Development
Beginning in 2002, a group of seven authors, working in collaboration with the company Plus Orthopedics GmbH (now Smith and Nephew), tried to improve upon existing systems by designing a neck-preserving prosthesis that would take anatomical requirements into account and provide biomechanically stable metaphyseal anchorage and load transfer. The result is the Nanos neck preserving short-stem implant (distributed by Smith and Nephew, Memphis, Tennessee, USA), which implantation will be described in this chapter.
For this cause 565 CT scans of patients under the age of 65 were taken to analyze internal cortical geometry and positioning of the femoral head center. In another 50 CT scans, the femoral neck was measured from a subcapital point to the lesser trochanter in order to produce optimal three-dimensional prosthesis shaping. The CT scans also yielded information about cross-sectional geometry for determination of the prosthesis sizes, which allow the femoral neck to be preserved due to their triple-taper cross-section while simultaneously providing maximum rotational stability with their enlarged surface area. Ranges for the
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Total Hip Arthroplasty
Figure 21.1: Design of NANOS
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Figure 21.2: Off-set reconstruction
centrum-collumn-diaphyseal (CCD) angle were established by analyzing radiographs and performing cadaver operations.6
The resulting prosthesis (Fig. 21.1) of these investigations combines the following special features: the position of the femoral neck in the prosthesis serves to restore the physiological joint geometry. The increase in prosthesis size is matched by a corresponding increase in the offset, while the polynomial curvature on the proximal-medial and distal-lateral sides adapts itself to the anatomical varus/valgus position so that leg length and offset can be restored even with the use of a monoblock (Fig. 21.2). The decision to focus on stability helped avoid the complications associated with modular neck systems, such as corrosion,7 migration,8 or breakage.9 Owing to its small and delicate shape, the prosthesis was named NANOS (dwarf).
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Indications/Contraindications
THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Short-stem implants were particularly developed for the treatment of active patients with primary and secondary osteoarthritis of the hip, femoral head necrosis, or dysplastic coxarthrosis. Although the implant only requires minimum bone resection since the cancellous bone around the metaphysis and the greater trochanter are retained, a good bone structure as well as sufficient primary stability at implantation are mandatory.10 The prosthesis is also designed to restore adequate joint geometry. The NANOS prosthesis is especially suitable for use in minimally-invasive procedures, while still leaving all the options open for the surgeon in terms of switching to a standard implantation. Contraindications are seen in extreme coxa valga or coxa vara and femoral neck deformities, which are likely to make implantation difficult or even impossible. In our experience severe osteoporosis and previous operations that failed to provide metaphyseal support, are also contraindications.10
Preoperative Planning
The operation, regarding the implant size, offset and position, is planned using a general pelvic X-ray and a two-plane X-ray (Figs 21.3 and 21.4). CT scans providing information to define medial and lateral contours, head position and head sizes are useful but not compulsory
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Figure 21.3A: Osteoarthritis. General X-ray of pelvis
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Figure 21.3B: Osteoarthritis. Axial X-ray of right hip Figure 21.3C: Osteoarthritis. Draft of prosthesis on AP X-ray of hip (preoperative planning)
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Figure 21.4: Preoperative planning of hip replacement with NANOS short-stem prosthesis using template to assess appropriate implant size
for preoperative planning (Figs 21.5A and B). Existing templates or design programs can be utilized. Depending on the system requirements, the aim is to achieve contact with the femoral calcar and the lateral cortex in the region of the tip of the prosthesis, on the AP view. In the axial beam path, the aim is to achieve proximal wedging in the ventro-dorsal region and close contact of the tip of the prosthesis in the region of the dorsal femoral cortical bone. The distal curved and beveled polished section of the prosthesis should reach a position distal to the lesser trochanter. During the surgical procedure, two-thirds of the femoral neck can usually be preserved on the medial side, leaving a zone for anchorage and load (Fig. 21.6). On the lateral side, a slightly more distal resection is usually necessary in order to provide a smooth transition between the beveled prosthesis shoulder and the lateral femoral neck. This still preserves the attachments for the muscles responsible for hip posture and movement.
The NANOS Short-stem Prosthesis
The NANOS femoral neck prosthesis has a triple taper design (Fig. 21.1). It is made from a forged titanium alloy and the proximal section is covered with a titanium plasma sprayed coating, which itself is provided with an additional calcium-phosphate layer for improved osseointegration. The neck area and the tip are highly polished. Ten sizes designed to complement each other and a clearly arranged set of instruments make it easier for the surgeon to intraoperatively determine and select the suitable implant.
SYNOPSIS OF MAIN FEATURES
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10 coordinated sizes with the 12/14 cone
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Extensive surface area contact in the calcar region for load transfer
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Cortical contact for support and for compensating loads in the varus direction on the distal-lateral side
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Titanium plasma sprayed (Ra 25 μm +/– 5 μm) coating and osteoconductive layer (Bonit®) in the proximal section for optimal osseointegration
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Polished distal tip to prevent osseointegration and stress shielding
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Tapered, polished femoral neck for optimized range of motion and prevention of impingement
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Titanium forged alloy (ISO 5832-3) for biocompatibility and strength
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case for storing the implantation instruments (Fig. 21.7).
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THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Figure 21.5A: CT scans of proximal femur to define medial and lateral contours, head position and sizes
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Figure 21.5B: CT scan of femoral neck region to define optimal cross-section
Figure 21.6: Axial saw cut through female femur with NANOS prosthesis
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Figure 21.7: Case for implanting instruments
Surgery
ANESTHESIA
We provide the patient with general and an epidural anesthesia at the level of Th8 in order to relax all muscles moving the hip. Conventional single-shot antibiotic cover should always be administered. We usually use IV Cephazolin® as prophylactic antibiotic, given at the time of induction.
POSITIONING AND APPROACH
A minimally-invasive anterolateral approach to the hip is used, provided in the chapter on MIS through an anterolateral approach.
RESECTION OF FEMORAL HEAD
After the joint capsule has been opened the femoral head is resected in a bone preserving manner: 1 cm subcapitally, perpendicular to the femoral neck. If necessary the resection angle can also be orientated 10 to 20° laterally to the greater trochanter (Fig. 21.8). Since it is desirable to preserve the circulous circumference of the anterolateral aspect of the femoral neck, the latter is protected by a wide chisel when the femoral extractor is inserted (Figs 21.9A and B). The transverse incision of the residual labrum facilitates the extraction of the femoral head, as does the use of an extraction-curette (Fig. 21.9C). If the resection is too close to the head, subsequently a resection of a plate of 0.5 to 1 cm from the femoral neck is required (Fig. 21.10).
CUP-IMPLANTATION
Standard instruments are used to ream the acetabulum. The wide ventral Hohmann retractor (HR) is left in position, a two-pronged HR is placed around the dorsal acetabular rim and another HR is placed above the caudal rim of the acetabulum (Fig. 21.11) Then the acetabulum is reamed until subchondral punctiform hemorrhages are visible (Figs 21.12A and B). This
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THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Figure 21.8: Femoral head and neck. Resection line can be orientated 10-20° laterally
Figures 21.9A to C: (A) After osteotomy of femoral neck. Head-extractor before insertion (B) Head-extractor inserted (C) Extraction-curette
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Figure 21.10: Resected femoral head. In this case, the resection line is too close to the femoral head–resection of plate (1cm) from femoral neck is needed
Figure 21.11: Femoral head removed. Acetabulum.
V: wide ventral HR is left in position
D: two-pronged HR placed around the dorsal acetabular rim
C: HR placed above the caudal rim of the acetabulum
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Figures 21.12A and B: (A) Smallest acetabulum-reamer (B) Reamed acetabulum
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THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Figures 21.13A to F: (A) ANA.NOVA® cup, coated with Bonit® (B) Implantation of cup (C) Rotation of cup (D) Impaction of cup (E) Impaction of cup (F) Impaction of cup
is preceded by complete resection of the soft tissue from the acetabular rim and the removal of any eventual dorsal and caudal osteophytes. A spongious graft taken out of the last reamer can be impacted into the acetabular notch. In cases of dysplasia a protrusion socket plastic or a lateral acetabular reconstruction can be indispensible. The NANOS stem can be combined with different cup systems. Therefore reaming should be performed as required by the system being used. We prefer, in dysplasia or revision, the use of the ANA.NOVA cup® (Intraplant GmbH, Medizinische Produkte, Grenzgasse 38a, 2340 Mödling, Australia) or a screwed cup. The ANA.NOVA cup® (Fig. 21.13A) is a special press-fit cup coated with Bonit® to promote osseointegration. It is 2% oversized compared to the reamer, which means an original cup size 50 is 1 mm bigger than the reamer size 50, and with its six wings, the flattened bottom and the rough Bonit® surface it finds a very stable press-fit.
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Figure 21.14: Impacted cup
Figure 21.15: Impaction of inlay/liner
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After reaming the cup implant is fitted to the inserter and inserted with the desired antetorsion and inclination—taking account of the zone that is secure in respect of dislocation (Figs 21.13B to F and 21.14). This task is facilitated by the use of an instrument set with a headless-impactor.
After insertion of the inlay (Fig. 21.15) the correct position has to be checked (Fig. 21.16). The leg extensions of the operating table are lowered by 20 to 30°. The operated leg is flexed, externally rotated by 90° and its knee is positioned above the knee of the other hyperextended leg (Fig. 21.17).
OPENING OF INTRAMEDULLARY CANAL
Using a medullary rasp and a small preliminary rasp, the course of the femoral neck and the transition between the metaphysis and diaphysis is carefully palpated through a central entry point in the femoral neck and opened up (Fig. 21.18). Protection of soft tissues should be provided by two HRs, one to protect the Gluteus medius muscle and another is placed over the medial femoral neck (Fig. 21.19A). Ideally, the tip of the preliminary rasp should reach the lateral cortex in order to produce the best possible prosthesis position (Fig. 21.19B). The
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THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Figure 21.16: Checking correct position of inlay/liner
Figure 21.17: Positioning of patient for reaming of femoral neck and implantation of prosthesis
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Figure 21.18: Model: Reaming of intramedullary canal
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Figures 21.19A to C: (A) Small curved rasp and Hohmann retractor (HR) positioning. V: HR placed over, D: HR to protect gluteus medius muscle (B) Small curved rasp in intramedullary canal (C) Rasp-size 0
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Figures 21.20A to C: (A) Model: Cancellous compactor (B) Compactor
(C) Compactor being impacted
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THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
medullary cavity is then widened using a prosthesis-shaped rasp (Fig. 21.19C). There should be no free space that is not filled by the prosthesis. Cancellous bone impactors are subsequently used to compact the cancellous bone, preventing it from being rasped out. The ribs of these compactors are arranged in a spiral to relieve the pressure in the medullary cavity (Figs 21.20A to C). The preparation is carried out in stages until the planned size is reached and the wedge-shaped compactor sits stably and in contact with the cortical bone (Figs 21.21A and B). The surgeon should be able to note the change in sound as the compactor
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Figures 21.21A and B: Impacted compactor
Figure 21.22: Compactor with trial head
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Figures 21.23A and B: (A) Control X-ray of seating of compactor; Not flush with femoral neck
(B) Control X-ray: Compactor flush with femoral neck
is struck. A trial head is fitted to the neck of the compactor (Fig. 21.22) and the head is reduced into the cup and the position is checked under the image converter with the compactor in situ (Fig. 21.23A). The size of the compactor and the planning are also checked. If the compactor is already flush with the osteotomy line but doubts about stability are still there, the next larger implant should be tried to avoid subsidence (Fig. 21.23B). To avoid fissures and fractures that could form as a result of its wedge, the compactor should only be advanced gently.
IMPACTION OF FEMORAL PROSTHESIS
The compactor is removed and the actual implant of the size of the last compactor is impacted into the same position. The compactor with the trial head as well as the prosthesis with the original head should always be repositioned carefully, since a short-stem prosthesis especially
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Figures 21.24A to C: (A) Model: NANOS (B) Prosthesis
(C) Impaction of prosthesis
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Figure 21.25: Impaction of definite head
Figure 21.26: Reduction of femoral head into cup
THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
transfers rotational and shear forces over a short distance (Figs 21.24A and B). Off-set and neck length are optimally adjusted by the use of appropriate head sizes. The cone is then cleaned carefully and the femoral head is fitted and gently tapped into place (Fig. 21.25). The femoral head is then reduced into the cup (Fig. 21.26). It should be noted that since the surface texture of the prosthesis is rasp-like, further slight subsidence of the prosthesis of 1 to 2 mm is possible; any greater subsidence is not acceptable. Mobility and stability have to be checked for the absence of any impingement (Figs 21.27A and B). Correct implant position is checked radiologically (Fig. 21.28). After liberal wound irrigation and the check for the absence of bleeding the wound is closed in layers (Figs 21.29 and 21.30).
Postoperative Management
Single-shot antibiotic cover and perioperative thromboprophylaxis are recommended.
Just as much consideration should be given to the individual pre- and intraoperative circumstances as to the general periods for soft tissue regeneration and osseointegration. The follow-up management is the same as that for tried and tested prosthesis systems.
In view of the soft tissue incision and the yet slight surgical trauma, we recommend a 4-week period of partial weight-bearing on the operated leg.
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Figures 21.27A and B: Assessment of range of motion: (A) flexion (B) external rotation
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Figure 21.28: Postoperative X-ray of the illustrated procedure
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Figure 21.29: After reduction—showing intact gluteus medius
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THA Using the NANOS Neck Preserving Stem Through an Anterolateral MIS Approach
Figure 21.30: Suturing of fascia lata
Results
Clinical results with similarly designed, femoral neck-preserving stems vary. Due to the recency of these systems most studies performed so far, could only assess follow-ups of five years at the most. The CUT prosthesis at Five-Year follow-up (ESKA Implants, Lübeck, Germany) indicated a survival rate of only 89%, with a high rate of aseptic loosening,11 noting a steep learning curve. The authors recommended that surgeons with minor experience with the system, operating on young patients, should use standard stems.12
Pipino's clinical results with the CFP prosthetic stem (Waldemar Link, Hamburg, Germany)3 were good-to-excellent in 96.6% of patients, with complete integration of the prosthesis occurring in 99% of cases.
Our results with the NANOS stem are more in line with those reported by Pipino, having neither stem-specific problems nor revisions. All but 1 patient experienced good-to-excellent HHS (Harris Hip Score) outcomes at 1-year follow-up; all patients with 3-year follow-up data experienced excellent HHS outcomes.10
The only radiological abnormality found, the flattening of the medial femoral neck cortex not impacting radiological positioning, was not unexpected, as this phenomenon was postulated in a 2007 analysis by Speirs et al.14 In that study, the authors developed finite element models of the NANOS in three different positions (one reproducing the intact hip center, one in increased anteversion and one in increased offset) and compared them with models of an intact, nonimplanted femur to evaluate the possibility for stress shielding with this short-stem prosthesis. They observed that although implant positioning had a negligible impact on cortical strains along the length of the diaphysis in comparison with the intact femur model, strain decreases of up to 95% were seen at the neck resection level. In comparison with the intact femur, larger decreases were noted with models on the medial than on the lateral side (95% vs 36%, respectively). All three implant models showed signs of characteristic proximal stress shielding. Although the authors concluded that these decreases in strain energy density may have consequences for longer-term bone remodeling, they did not believe small changes in stem placement would significantly impact internal femur loading following bone ingrowth. Longer follow-up will be necessary to fully gauge the impact of these radiological findings in the current cohort of patients.
Illustrative Case
A very active 62 years old man with coxarthrosis following CAM-impingement (Figs 21.31A and B) presented with flexion deficit and with long lasting pain on rotation.
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Figures 21.31A to E: (A) Case: General X-ray of patient with coxarthrosis in right hip after CAM-Impingement (B) Case: Preoperative axial X-ray (C) Case:Preoperative planning of short-stem prosthesis (D) Case: Postoperative AP X-ray, right hip (E) Case: Postoperative axial X-ray
TREATMENT
The patient was treated with a NANOS prosthesis and an ANA.NOVA® cup, minimally-invasive implanted through the anterolateral approach (Figs 21.31C to E) Anterior capsular resection and resection of acetabular as well as neck osteophytes were carried out. A neck preserving short-stem implantation was performed. The inlay was a “delta ceramic” 36 mm, and the head was a “delta ceramic” 36 mm also both by Ceramtec-Biolox (CeramTec GmbH, Plochingen, Germany). The skin incision measured 7 cm. At 6 months follow-up the patient presented without complaints and full mobility.