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DEVELOPMENT OF HIP ENDOPROSTHESIS FOR TREATING PATIENTS WITH FEMORAL NECK PSEUDARTHROSIS
Varfolomeev D.I., Samoday V.G.
Voronezh State Medical University named after N.N. Burdenko, Voronezh, Russia
Fractures of the femoral neck are the serious problem of modern
traumatology and orthopedics. Such fractures are presented as single,
associated or multiple injuries to the locomotor system. Femoral neck fractures
and their consequences are common for patients of older and senile age.
According to the literature, the rate of false joints is up to 80 % for
conservative treatment and 30-40 % for surgical one (osteosynthesis) [1, 2, 3].
At the present time, the various types of efficient reconstructive
surgery for false joints of the femoral neck have been developed: internal fixation
(also in combination with bone and muscular plastic surgery), osteotomy for the
proximal femoral bone and others [4, 5, 6]. But long time of union, presence of
osteoporosis, anatomic and functional changes in an injured joint often hinder
such surgical interventions [7]. The rate of non-union and development of
aseptic necrosis of the femoral head are 10-20 % after different types of
osteosynthesis (Rashan A.) [8].
Some authors believe that endoprosthetic replacement allows fast restoration
of supporting ability of the extremity and decreasing pain in the articular
region. In some cases the treatment is possible only with joint replacement,
for example, for femoral head aseptic necrosis, when the head destructs
actually.
Endoprosthetic replacement for false joints of the femoral neck relates
to difficult cases of primary joint replacement. This fact is mainly determined
by significant shift of the femoral bone towards proximal direction, because of
contraction in the muscles surrounding the hip joint, in cases with absent
previous surgery, and also in presence of significant amount of scars in the
joint region. Such patients often demonstrate flexion-adduction contracture in
the hip joint that sometimes require for tenotomy of adductor muscles [7].
For patients with femoral neck false joints the endoprosthetic
replacement requires for restoration of extremity length and optimal
relationships in the artificial joint (Fig. 1).
Figure 1. X-ray image of the patient A. with femoral neck pseudarthrosis
Because of significant shortening of the extremity, restoration of
previous length is associated with increasing rates of soft tissue injuries owing
to necessary extension of surgical approach and removal of scars around the
joint. Significant single-moment hyperextension of contracted and hypertrophied
muscles leads to intense postsurgical pain in the joint, with increasing risk
of postsurgical complications and decreasing quality of life. It is necessary
to note a possible traction injury to the sciatic nerve that requires long term
treatment.
Therefore, endoprosthetic replacement of false joints of the femoral
neck requires the decrease in traumatic potential of surgical intervention and
restoration of optimal relationships in the artificial joint.
The objective of the study – to develop the original hip prosthesis for improving outcomes of joint replacement in patients with femoral neck pseudarthrosis.
MATERIALS AND METHODS
The original hip joint endoprosthesis has been developed for decreasing the traumatic effect of surgery and gaining the additional possibilities for correction of extremity length and antetorsion of the neck of the artificial joint [10]. The technical executability and practical implementation were estimated with the service model (made of ABC plastic) and 3d printing techniques (Fig. 2).
Figure 2. The hip joint endoprosthesis (the functional model): a) in the assembled condition: 1 – neck, 2 – stem; b) and c) in the disassembled condition: 3 – a pin, 4 – reducing component, 5, 7 – a roller, 6 – constant magnet, 8 – covers, 9 – right and left parts of the stem
The model has been developed on the basis of the chair of
traumatology and orthopedics, Voronezh State Medical University named after
N.N. Burdenko.
The endoprosthesis consists of the acetabular component, an insertion
piece, the spherical head, the neck and the stem of the endoprosthesis. CLS
Spotorno (Zimmer) was used as the analogue for the endoprosthesis stem.
The basis of the endoprosthesis neck has the cylindrical shape with
internal thread and the projecture in the lower part. The upper part of the
endoprosthesis has the cylindrical-shaped channel with the grooves, where the
bottom of the neck is situated, with the projecture of the neck in one of the
grooves. The grooves conjoin into the single grove in the lower part of the
channel (Fig. 3).
Figure 3. Hip joint endoprosthesis, top view: 1 – neck, 2 – stem, 3 – groove, 4 – ward in a groove
The middle part of the stem has the cavity, which is situated along the
axis of the stem. It includes the reduction unit. The lower part of the stem has
a similar (the second) cavity including a rod with magnet and the locking
spring. In the free position the magnet is locked by the spring into the
special groove of the stem that prevents possible rotation. The channel of the stem
and both cavities are connected with via openings, with the upper via opening
containing a pin, which is screwed into the thread of the basis of the neck.
Therefore, the endoprosthesis stem includes the mechanism for
transmitting the rotation from the magnet (located in the lower part of the
stem) to the pin (in the upper part of the stem) by means of the reduction unit.
The total ratio of the reduction unit (used for development of the model) is
1:298, i.e. one rotation of the pin is 298 rotations of the magnet. It gives
the significant increase in the force applied to the pin as compared to the
force applied to the magnet. All components of the endoprosthesis are made of paramagnetics.
The acetabular component, the insertion part and the head present the common
elements of the modern endoprosthesis. The covers of the cavities can be fixed
by means of mechanical jamming in the appropriate grooves of the stem or by
means of welding. The reducing unit presents a common microreductor, which is
used in micromotors. It consists of the set of straight-toothed gears.
The neck-diaphysis angle of the endoprosthesis is 125-135 degrees. For
old injuries to the femoral neck it is appropriate to use the implants with the
neck-diaphysis angle of 125 degrees.
The endoprosthesis has 3 types of the angle of antetorsion of the
femoral neck: 0°, +8°, -8°, in similar fashion with the modern endoprosthetic
implants with module necks. The increase in moving the neck out of the
endoprosthesis stem is possible only for the distance of 1-3 cm. Further moving
out is unpractical owing to the increasing risk of metal construct fracture
(Fig. 3, 4).
Figure 4. Functional capabilities of the endoprosthesis: a) anterior view (possible variants of change in antetorsion of the endoprosthesis neck), b) lateral view (possible change in limb extremity)
The principle of operation of the offered endoprosthesis is described
below.
During surgery for patients with old femoral bone injuries with
significant shortening of the lower extremity and upward shift of the proximal
femoral bone, the neck of the endoprosthesis is placed in the manner of minimal
length of the whole femoral component. Therefore, the minimal length of the
prominent part of the neck is achieved with the possibility of low traumatic
installation of the endoprosthesis into the position with minimal increase in
the extremity length. The increase in the extremity length is achieved during
the postsurgical period.
During the postsurgical period the correction of the extremity length
and femoral neck antetorsion are performed with the magnet system for “expanding”
implants [11], which are used in pediatric oncology (for increasing the
extremity length, when a child grows) (Fig. 5).
Figure 5. The lower extremity placed into the magnet coil: 1 – electromagnetic coil, 2 – direction of rotation of the extremity
After surgery in the initial position, the prominent part of the basis
of the neck is located in the single groove, with a possibility for rotation of
the neck within the sector limited by the prominent parts of the stem (Fig. 2,
3, 5). The operated lower extremity is placed into the magnet field in the
manner with location of the system in the middle one-third of the femur. A
doctor with his/her hand performs the mechanical turning of the patient’s leg
up to achieving the desired angle (the angle of neck antetorsion). Then the
magnet system is activated and the electric magnets rotate around the lower
extremity. This manipulation is inappropriate during X-ray control for
estimation of increase in the extremity length. For decreasing the extremity
length it is necessary to initiate reverse rotation of the magnets of the
system, where the patient’s leg is situated.
The stand with the rotating electric magnets was developed with use of
3d printing and was used for estimation of a possibility of the declared
functions performed by the endoprosthesis (Fig. 6).
Figure 6. The test bed with the demonstrative sample of the
endoprosthesis: 1 – neck, 2 – stem, 3 – magnet, 4 – magnet rack, 5 –
foundation, 6 – electric motor; the arrows show the direction of magnet
rotation
The stand consists of the basis including the micro electric motor, and the mobile plug with two constant magnets on the ends and the endoprosthesis stem between the magnets. The rotation of the magnets influences on the constant magnet (the position 6 in the figure 2b) in the endoprosthesis stem. It gives rotation of the magnet with transmitting the torque to the pin (through the reducing unit), which pulls out (or pulls in) the endoprosthesis neck depending on the direction of rotation of the magnets.
RESULTS AND DISCUSSION
The experimental studies have shown the possibility of change in the
length of the prominent part of the implant neck and antetorsion under
influence of magnetic field on the femoral component of the endoprosthesis.
The pathology of the hip joint with absolute or relative shortening of
the lower extremity is certainly not limited by only false joints of the
femoral neck. The list of such diseases includes dysplastic coxarthrosis, Legg-Calve-Perthes
disease, consequences of hip joint injuries, various effects of the proximal
femoral bone (during multi-stage revision interventions) and others.
During attempting the leg traction during surgery, a distraction injury
to the sciatic nerve appears, especially in the fibular part. Such injuries require
for subsequent long term treatment. It is sometimes impossible to perform the
full restoration of the extremity length. The significant stretching of soft
tissues causes the intense postsurgical pain. The use of the developed
endoprosthesis prevents the necessity for single-stage restoration of the
required length of the extremity. It is appropriate to do it within several
stages with gradual increasing the length, for example, 1 cm per week of rarer.
Such lengthening of the extremity can create the possibility for smooth
extension of scars and for preservation of adequate muscular tension.
Since the endoprosthesis allows changing the antetorsion of the length
of the extremity, it can correct surgeon’s errors to some extent. So, for
recurrent dislocations of the endoprosthesis with excessive anteversion of the
cup it is possible to decrease the femoral neck antetorsion to neutral value, thereby
performing non-invasive decrease in the possibility of dislocations.
Therefore, during the postsurgical period this endoprosthesis allows
correcting the intrasurgical errors, maximal restoring the rotation center and
achieving the maximal range of movements in the joint.
Since the endoprosthesis neck is without rigid fixation to the stem, the
mechanical wear takes place and metal particles appear in the place of
connection. As result, the alloys must have the maximal wear resistance.
CONCLUSION
During the recent years the researchers have been developing the various implants with ability to change the shape and configuration during surgery (plates with shape memory) and after it under influence of the various factors (usually, electromagnetic fields). The use of the developed endoprosthesis with non-invasive changes in the configuration and the functional capabilities will provide the maximally precise setting of the geometrical parameters of the endoprosthesis during surgery and after it and, as result, will decrease the traumatic potential of intervention and amount of complications and will improve the quality of life after joint replacement.
REFERENCES
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