Custom/patient-specific 3D-printed metallic porous spacer for the first metatarsophalangeal joint (MTPJ) with bone loss to restore length, alignment, and function of the first ray.

The first MTPJ is a ball and socket joint that consists of the articulation between the concave base of the first proximal phalanx and the convex 1st metatarsal head.

The sesamoids are contained within the flexor hallucis longus tendon and articulate with the plantar first metatarsal head.

The normal range of motion for the first MTPJ varies, ranging from 65 to 110 degrees of dorsiflexion and 23 to 45 degrees of plantarflexion. ^,

When performing first MTPJ fusion, the final position of the great toe should allow for heel rise during the late-stance phase of gait ^, ( Figs. 19.1 and 19.2 ).

Picture showing the ideal position of the hallux after first metatarsophalangeal joint fusion for heel off.

(CAD images reprinted with permission from restor3d, Inc, Durham, NC.)

Demonstration of heel off with a fused first metatarsophalangeal joint.

(CAD images reprinted with permission from restor3d, Inc, Durham, NC.)

The normal length of the first metatarsal is about 6.5 cm. ^, Shortening of the first metatarsal and first ray can lead to jamming of the first MTPJ and inhibits the ability of the first ray to propulse properly during the push off phase of gait. Alteration of the first metatarsal length has also been linked to the formation of hallux valgus and hallux limitus. ^,

Osseous deficits and shortening of the first ray may be secondary to:

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  Nonunion after primary first MTPJ fusion

  
  
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  Avascular necrosis (AVN): idiopathic, traumatic, or iatrogenic causes

  
  
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  Failed first MTPJ joint implant arthroplasty

  
  
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  Failed Keller arthroplasty

  
  
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  Osteomyelitis/septic arthritis.

Primary first MTPJ inherently causes a degree of first ray shortening. Aggressive reaming can cause thermal necrosis, which may increase the risk of nonunion—5% to 10% of primary first MTPJ fusions result in nonunion. During revision of first MTPJ nonunion, all nonviable bone requires resection, which will lead to further shortening.

Overzealous resection in Keller arthroplasty will result in iatrogenic shortening of the first ray, which can lead to functional and biomechanical problems.

Vascular insult may occur during head osteotomies of the first metatarsal, particularly if dissection is excessive and/or lateral release is concurrently performed. AVN may result if the vascularity to the first metatarsal head is not reestablished in a timely fashion.

First MTPJ implant arthroplasty inherently requires significant bone resection. Furthermore, implant material such as silicone has been associated with foreign body reaction, synovitis, and implant failure. ^, Aseptic loosening may also result in implant failure, which will often require revision. Removal of the implant during revision will result in a substantial osseous deficit.

Osseous deficits that result in a short first ray alter the propulsion phase of gait, which may cause pain, deformity, and biomechanical dysfunction. ^,

The degree of shortening that can be tolerated is variable from patient to patient.

Localized pain

Adjacent metatarsal pain (transfer metatarsalgia)

Deformity of the hallux (cock-up, varus, valgus)

A nonfunctional hallux (floppy toe)

A visible length difference between the hallux and the second toe

Weak propulsion during gait examination

Functional imbalance of the forefoot and compensatory changes in gait cycle. ^{, ,}

Radiographs:

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  Nonunion may be represented by a lack of callus and/or trabeculation along the osteotomy or fusion site, bony sclerosis, change in alignment, subluxation/dislocation of the capital fragment, and hardware failure.

  
  
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  AVN may be progressive and findings may range from osteopenia to sclerosis and cortical collapse, to eventual destructive changes to the joint ( Fig. 19.3 ).

  
  

  
  

  
  

  

  
  

  
  

  Exposure of the first metatarsophalangeal joint showing osteonecrosis and collapse of the first metatarsal head.

  
  
  
  
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  Loosening and failure of implant arthroplasty are represented by periprosthetic lucency, poor apposition at the bone-implant interface, and a change in implant position with serial radiographs. Loosening may progress to periprosthetic fracture and implant dislocation. ^,

  
  
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  Osteomyelitis: osteopenia, radiolucency, lytic changes/cortical erosion, trabecular destruction, bone necrosis, Brodie abscess, sclerosis, and periosteal reaction.

Advanced imaging (CT/MRI):

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  MRI is useful for the diagnosis of infection and is most sensitive for early diagnosis of AVN. MRI with contrast provides a better estimation of blood flow. If hardware is present MRI may not be useful.

  
  
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  CT may be more helpful in staging AVN, deciphering between fusion and nonunion, and implant loosening. CT is often more helpful in surgical planning and procedure selection.

  
  
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  Currently, CT is the imaging modality of choice if 3D printing technology is being utilized.

Other testing:

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  Noninvasive vascular testing should be considered if there is concern for vascular compromise.

  
  
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  Infectious and bone metabolism workup should be obtained in cases of nonunion and radiolucency/pathologic fracture surrounding the first MTPJ implant arthroplasty, including white blood cell count (WBCs), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), vitamin D, parathyroid hormone (PTH), and thyroid-stimulating hormone (TSH).

  
  
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  Joint aspiration can be considered in cases where there is increased suspicion for infection.

Shoe modification: wide, rigid toe box

Orthotics and/or metatarsal pads

Immobilization

Rest, ice, compression, elevation (RICE) therapy and nonsteroidal anti-inflammatory drugs (NSAIDs).

In situ fusion: results in acute shortening and problematic biomechanics, is aesthetically less pleasing, and could cause soft tissue compromise. Alternative surgical techniques should be used if the osseous deficit is >1 cm.

First MTPJ fusion with bone block: graft incorporation and fusion at two sides of the graft is required, which may decrease time to union and the rate of union. The current consensus is that the maximum size of the graft is 2 to 2.5 cm, therefore for larger deficits shortening will result. Autograft may be superior to allograft; however, autograft is associated with donor site morbidity and a complication rate of nearly 30%.

Distraction osteogenesis: requires an extended time in an external fixator, which is associated with pin site infections and poor patient compliance. The quality of the regenerated bone is of concern, especially in smokers and diabetics.

Vascularized free fibula: technically challenging and the operative time can be extensive. Donor site morbidity is further concerning.

Masquelet technique: requires staged procedures and drainage from the antibiotic spacer may cause soft tissue issues.

Partial first ray amputation: amputation should be a last resort and reserved primarily for limb salvage in the setting of severe infection, trauma, or failure of multiple revisions. Amputation results in permanent loss of first ray function and is fraught with secondary symptoms, which may lead to further minor or major amputation.

Titanium has been shown to have osteoconductive properties ^, and is biologically inert, therefore titanium is the material of choice.

A variety of proprietary porous structures are available. The senior author currently prefers a gyroid structure.

Pore size:

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  Pore size has been described in the literature from 300 to 1000 μm. ^, The senior author recommends a size of 1000 μm, as this is within range and allows for the most bony ingrowth.

  
  
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  Degree of porosity: the percentage of porosity is inversely proportional to implant strength. This should be considered during implant design. Typically, the senior author prefers 75% porosity, which allows bone graft retention and bony ingrowth potential while having adequate strength.

The senior author recommends a pill-shaped implant with two convex surfaces ( Fig. 19.4 ), which promotes a larger surface area of fusion and enhanced interposition of the implant to metaphyseal bone. In the senior author’s experience, an implant with two convex surfaces also allows easier positioning and appears inherently more stable compared to other configurations. This design has also been advocated for in the literature regarding allograft and autograft bone block arthrodesis. ^,

A pill-shaped implant with two convex surfaces.

(CAD images reprinted with permission from restor3d, Inc, Durham, NC.)

The porous surface is in direct contact with the adjacent bone and areas of smooth/polished surfaces to prevent soft tissue ingrowth and contracture.

To account for intraoperative variability, the authors prefer to have three implants available which range from nominal and ± 5% to 10% of the length of the nominal-sized implant.

The length of the implant is case specific and based on the proposed resection level. The length of osseous deficit after resection of nonviable bone and preparation for fusion determines the nominal length. Custom cutting guides may be created and used to ensure the resection is accurate.

The diameter and radius of the convex surfaces are patient specific and based on the size and diameter of the first metatarsal and hallux at the proposed resection level. Reamer size availability should also be taken into account. The authors prefer to keep the diameter and radius of the convex surfaces constant among all implants. It is the senior author’s opinion that the diameter of the implant should ideally be within the confines of the cortices to allow for bony overgrowth around the implant. In other words, the implant diameter is equivalent to the diameter of the inner aspect of the cortex as determined on axial CT views ( Fig. 19.5 ).

Axial CT scan showing the inner diameter of the cortex. It is the senior author’s opinion that the inner cortical diameter is the ideal size for the implant to allow for bony overgrowth around the implant.

The implant can be designed with screw holes to allow direct implant-to-bone fixation; however, the more screws used, the less surface area available for bone ingrowth and fusion.

Off-the-shelf items required:

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  Convex (or cone) conical reamers sized according to the implant size.

  
  
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  A 3.5-mm dorsal locking plate to provide initial stability and maintenance of alignment while separate from the implant in case hardware removal is needed.

  
  
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  Solid/cannulated screws for direct implant-to-bone fixation if desired.

Positioning: supine with an ipsilateral bump.

Anesthesia: general and regional.

Approach: standard dorsomedial approach along the medial side of the extensor hallucis longus (EHL), in the interval between dorsal proper digital nerve and EHL; however, consider the placement of existing incisions.

Technique:

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  Expose the first MTPJ with adequate visualization of the proximal phalanx and first metatarsal ( Fig. 19.6 ).

  
  

  
  

  
  

  

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