Cannulated Hemi Arthroplasty of the 1st MPJ |
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By Stephen J. Medawar, D.P.M.
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Residency Question of the Week:
Why do residents prefer to work with Vilex Screws? (See answer below). |
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Degenerative joint disease of the 1st MPJ is a painful and debilitating condition. Pain of the great toe joint often leads to altered lifestyles in an attempt to deal with the pain of each step.
There are many conservative and surgical treatments available to attempt to relieve discomfort. Once the practitioner has exhausted conservative care including change of shoe gear, oral medications, injection therapy and functional orthosis; surgical intervention may be considered.
There are several surgical options to consider in approaching the painful degenerative 1st MPJ. This paper will concern itself with the implant arthroplasty of the first MPJ using Cannulated Hemi Implants by Vilex to resurface the base of the proximal phalanx.
PATHOLOGY
Degenerative joint disease in the arthrodial joints follows the same general process. The loss of hyaline cartilage from trauma, systemic factors or faulty mechanics is the initiating factor. The deterioration of the cartilage leads to subchondral sclerosis and bone cyst formation. The synovial membrane then becomes inflamed and joint fluid increases. As this progresses, the joint capsule becomes thicker and the osteophytic proliferation begins to limit joint motion (Fig. 1). This increase in bone formation and subsequent decreased joint motion is often painful. Pain in the joint leads to compensatory gait. This includes increased lateral metatarsal load, external rotation of the leg at the hip joint and reduced stride length.
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Fig.1: Typical DJD changes of 1st MPJ |
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The goal of implant arthroplasty is to stop the degeneration of the cartilage, which is the source of pain. The purpose of use of the cannulated hemi implant is to resurface the base of the proximal phalanx, which will stop the arthritic cascade. The design of the implant mimics the radial curve of the base of the proximal phalanx. The implants congruent surface is made of chromium cobalt, which is highly biocompatible and has excellent wear properties. The thickness of the implant minimizes the amount of bone resected at the base of the proximal phalanx, preserving important intrinsic muscles to help restore normal joint kinematics. This is vital to minimize lateral metatarsalgia which is so common after a Keller arthroplasty, silastic implant and joint fusion.
PROCEDURE
The surgical approach is made from a dorsomedial skin incision centered over the 1st MPJ. The incision begins well proximal to the metatarsal head and extends to the diaphysis of the proximal phalanx. This is necessary since exposure while placing the implant is crucial for alignment.
The same incision line is used to incise the periosteum and joint capsule. Carefully reflect the periosteum using the freer elevator and scalpel to preserve the capsular layer. All patholigic bone of the first metatarsal head and base of the proximal phalanx is removed with ronguer and power oscillating saw. My preference is to irrigate the wound after each step with antibiotic irrigation. This keeps the tissues hydrated, clears debris and minimizes the chance of post-op infection.
The base of the proximal Phalanx is resected perpendicular to the shaft, taking 3-4 mm of bone distal to the dell of the phalangeal base(Fig. 2). When dissecting the base of the phalanx from the shaft, care must be taken to preserve the FHL and FHB tendon attachment. (Fig. 3)
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Fig. 2: Resection of base proximal phalanx |
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Fig. 3: Careful excision of base |
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Using the Vilex trial sizer, select an implant which will cover the cortical margins of the base. An implant which is too small will “bulldoze” over time into the phalanx, and an implant too large will be palpable thru the skin. Select the proper size implant.
The safest way to place the .062 guide wire is to apply the implant upside down on the proximal phalanx base, maintaining cortical purchase, and drive the guide wire into the shaft using the implant as a guide. This will insure both centering of the implant and flush positioning of the Vilex implant. (Fig 4 and 5)
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Fig. 4: Upside down CHI NOT aligned
for cortical purchase |
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Fig. 5: CHI use to insure congruent fit |
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Once the wire is driven into the phalanx at least 3 cm, then the implant can be removed from the guide wire (Fig. 6). Simply, apply the implant down the guide wire in its correct position and use the 2 prong driver to insert the implant. I feel that the phalanx should be trephined 5 mm to start the Vilex implant placement (Fig. 7).
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Fig. 6: CHI used for propert fit and
insertion of guide wire |
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Fig. 7: Trephined starter hole approx.
5 mm deep |
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Using the AO technique to drive the implant home is useful. Two turns clockwise, ½ turn counter clock wise will clear the threads and allow the implant to seat and align with no difficulty.
When placing the implant, PULL the hallux distally in the flexed position. Merely rotating the toe plantigrade may not give you the room necessary to clear the 1st met head. Remove the guide wire and load the forefoot, check the range of motion for impingements and restrictions (Fig. 8).
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Fig.8: Properly inserted Vilex implant |
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One must consider that the soft tissues and sesamoids will have adapted to the rigid preoperative condition. Releasing the sesamoids with a McGlamery scoop, freer elevator or key elevator is mandatory. Consider a partial release of the fibular sesamoid collateral ligament as this is often tightly adapted.
Close in layers and use a post operative injection of Dexamethasone Phosphate 4mg in 5cc .5% Sensorcaine Plain. This will minimize inflammation enabling the patient to start early range of motion.
Personally, I confine the patient to a post operative shoe for one month. Keep the foot elevated to hip height 2 hours per 15 minutes of dependency for one month post operatively to minimize scar formation, infection, and pain.
DISCUSSION
The benefits of the Vilex CHI (Cannulated Hemi Implant) prosthesis are clear. Having placed more than 80 CHI implants over the last four years, I have seen no transfer metatarsalgia.
This implant preserves the function of the intrinsic musculature.
Restoring 1st MPJ stability and kinematics enables the patient to propel more normally. This is evidenced even when only 30° of dorsal flexion is available (Fig. 9).
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Fig.9: Post-operative range of motion x-ray |
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The range of sizes and superior screw design insures the Vilex implant will not migrate in normal bone. The large diameter flutes of the Vilex implant make it ideal for implant revision when replacing other failed prosthesis The chromium cobalt material is extremely biocompatible and causes no irritation when articulating on degraded opposing surfaces.
Implants manufactured using Titanium are available for patients allergic to nickel. Care should be taken to ensure adequate bone resection of the proximal phalanx and dorsal metatarsal head to allow normal motion.
SUMMARY
In the surgical treatment of hallux rigidus, the practitioner should remain conservative by starting with a Cheilectomy.
If the x-ray and preoperative findings strongly suggest a Cheilectomy is not feasible, then the option of arthrodesis, arthroplasty and implantation can be discussed. The use of implants as a first line surgical treatment still allows for Keller arthroplasty or arthrodesis in the future. The same cannot be said for the reverse. Intrinsic muscular wasting and limited function plague the latter procedures. The use of the Vilex cannulated hemi implant leaves the patient and practitioner options for future care if the need presents itself.
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Answer to the Residency Question of the Week:
The cutting flutes are extremely sharp. It takes very little effort to insert the screw, reducing the risk of fracturing your osteotomy. All Vilex screws have a low head profile and have hand-sharpened triple cutting tips.
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The main business of Vilex is orthopedic and podiatric bone surgery. Vilex® offers a broad range of cannulated screws, locking and conventional plates, small joint implants, a subtalar Arthroereisis implant, and ring and mono-rail external fixation devices. In addition, through United American Medical (UAM, acquired in 2001), Vilex® offers a whole array of power equipment such as wire drivers, drills, and oscillating/reciprocating saws. |
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Vilex® also offers saw blades, guide wires, drills and burrs. All Vilex Internal Fixation products are manufactured in the USA. Call 800.521.5002 for more informatin of visit www.vilex.com today.
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