35 Pyrocarbon Scaphoid Implant Allowing Adaptive Mobility in Proximal Scaphoid Pseudarthrosis

10.1055/b-0034-80600

35 Pyrocarbon Scaphoid Implant Allowing Adaptive Mobility in Proximal Scaphoid Pseudarthrosis

Pequignot, Jean-Pierre, Allieu, Yves

Long-standing scaphoid nonunion with pseudarthrosis and avascular necrosis of the proximal part of the scaphoid is a difficult problem to treat, with haphazard results.1 Over time this condition will result in degenerative periscaphoid arthritis that progressively involved the radiocarpal and midcarpal joints with secondary carpal collapse. This has been termed a scaphoid nonunion advanced collapse (SNAC) wrist,2 ^, 3 which is secondary to the loss of carpal alignment.4

This chapter describes the implantation of an adaptive proximal scaphoid implant (APSI), which replaces the proximal part of the scaphoid. It is designed to treat periscaphoid arthritis and to prevent further deterioration and carpal collapse by restoring the height of the proximal carpal row.

▪ The Adaptive Proximal Scaphoid Implant

This proximal scaphoid implant is manufactured from pyrocarbon ( Table 35.1 ). Due to its ovoid shape and its material, it allows the implant to adapt to the changing carpal geometry with proximal row motion.5 In the frontal plane the curve of the scaphoid fossa of the radius is relatively small, whereas in the sagittal plane the scaphoid fossa forms an ovoid with a larger anteroposterior diameter ( Fig. 35.1A,B ).

By rotating on the two axes during radial and ulnar deviation and during wrist flexion and extension, the implant imitates the movements of the proximal scaphoid in a synchronous fashion with the carpal kinematics6 ( Fig. 35.2 ). In addition, because of this three-dimensional reorientation during wrist motion, the implant remains stable without the need for periprosthetic encapsulation or fixation with the distal scaphoid ( Fig. 35.3 ). Thus this implant is both mobile and stable. It is available in three sizes whose parameters vary proportionally while retaining the same radii of curvature.

▪ Implant Material

The APSI is manufactured from pyrocarbon “material,” which has been shown to be very biocompatible7 ^– 8 ( Table 35.1 ). Extremely hard-wearing and chemically inert and generates no wear on the bone. Its coefficient of friction against bone and cartilage is very low, which allows the implant to slide against the surrounding cartilage and ligaments to follow the path of least resistance against the deformable walls of its cage. Because there is no adherence to the surrounding soft tissue there are no adhesions that may act as leverage points to cause a dislocation. Its elastic modulus (Young modulus), is nearly identical to that of bone, leading to good patient tolerance and minimal bone wear.9 A large difference in the elastic modulus between an implant and bone or cartilage can create a point of adherence or wear that disturbs the sliding-rolling motion of the implant, and may result in dislocation ( Fig. 35.4 ).

**Table 35.1** **Mechanical Properties of Pyrocarbon Compared with Bone and Other Materials**
		Mechanical Properties
Pyrocarbon Properties	Young Module Gpa	Breaking Stress Mpa	Elongation Stress
Cortical bone	20	150	1–3
Pyrocarbon	25	500	1.6–2.1
Silicone	0.004	10–12	700–800
HD polyethylene	1.05	22–31	450
CoCr	200–240	600–1500	8–50
Stainless steel	193–210	480–1300	40
TA6V	102–114	860	8
Abbreviations: CoCr, cobalt chromium; Gpa, young module; HD, high density polyethlene; Mpa, megapascals; TA6V, titanium allay.

**Fig. 35.1** Frontal anatomical cuts. **(A)** The proximal scaphoid presents a small radius of curvature in the frontal plane (r), and a large curvature (R) in the anteroposterior plane. **(B)** Replacement with trial implant with same radii of curvatures.

▪ Indications

The pyrocarbon implant is indicated as a palliative procedure for maintaining carpal height in cases of chronic scaphoid nonunion with an avascular proximal pole, with or without a scaphoid nonunion collapse pattern.10 It has also been used for radial styloid osteoarthritis due to a chronic scapholunate advanced collapse (SLAC) wrist. SLAC wrist contraindications would include active infection, inadequate soft tissue coverage, and marked radiocarpal instability or ulnar translocation.

**Fig. 35.2** In radial deviation the scaphoid is flexed and (r) is perpendicular to the forearm axis. In ulnar deviation (R) the scaphoid extends.

**Fig. 35.3** **(A)** Motorcycle accident in a 28-year-old patient, manual worker, 2 years after an initial surgery. **(B)** Dynamic x-rays 8 years after surgery, which included a limited styloidectomy. The patient maintained a good result and the same job. The proximal scaphoid presents a small radius of curvature in the frontal plane (r) and a large curvature (R) n the anteroposterior plane.

**Fig. 35.4** An implant placed between two bones O1 and O2 is under significant compressive force. **(A)** With a high coefficient of friction, the implant cannot slide or translate, which leads to displacement. **(B)** With a low friction coefficient, the implant slides and turns on itself, which prevents displacement.

STT advanced arthrosis or Radiolunate degenerative change

Bone loss secondary to radial comminuted fractures

▪ Surgical Technique

The dorsal approach was used in the majority of cases, except in four patients who had previous surgery through an anterior approach. A dorsal incision is made centered over the Lister tubercle, which allows one to raise the dorsal radiocarpal ligament in a horizontal V-shaped manner, with its apex at the triquetrum. A capsulotomy is performed to expose the dorsal edge of the radius and the head of the capitate with the dorsal intercarpal ligament. The ununited proximal scaphoid pole is excised at the nonunion site. At this point a radial styloidectomy is performed. A trial implant is then inserted both to assess the volume of the cavity and to allow the surgeon to assess its mobility in radial deviation and stability in both the frontal and anteroposterior planes under fluoroscopic guidance.

It is preferable to undersize the implant rather than to attempt to correct a marked loss of carpal height or a severe dorsal intercalated segment instability (DISI) deformity. In cases of uncertain stability, particularly with an anterior and posterior drawer maneuver, the dorsal intercarpal ligament should be reinserted on the distal scaphoid using a bone anchor, as described by Szabo11 and Moritomo12 ( Fig. 35.5 ). In selected cases of instability with a severe DISI deformity or with ulnar translation, we have performed a reattachment or application of the radioscaphocapitate ligament via an anterior approach using bone anchors ( Fig. 35.6 ).

Microdrilling was performed in 12 cases where the subchondral bone of the radius was very sclerotic.13 We performed a minimal radial styloidectomy in 16 patients with preoperative signs of radial styloid impingement and reduced radial deviation. Seven cases had a significant DISI greater than 20 degrees, which was corrected by a dorsal capsulodesis. A postoperative cast was applied for 3 weeks, followed by a removable splint for approximately 2 months.

**Fig. 35.5** Right dominant hand in a 42-year-old woman (rock climber). **(A)** A scaphoid nonunion advanced collapse with marked dorsal intercalated segment instability of 20 degrees. **(B)** Correction following insertion of an adaptive proximal scaphoid implant and a combined dorsal intercarpal ligament reconstruction. **(C–D)** Normal use and function of her wrist 9 years postoperatively.

▪ Results

We initially reported our results on 25 patients who had received the APSI implant in November 2000.14 This retrospective series included 25 cases that were examined at an average of 6 years postsurgery (range, 3 to 10 years) by an independent observer. There were 14 cases of SNAC wrist, 10 cases of SLAC wrist, and one case with carpal collapse and silicone synovitis following insertion of a silicone partial scaphoid implant. The indication for surgery in all cases was pain. Postoperatively, the pain resolved in 60% of the patients. In 28% of cases pain was only present with exertion. Thus 88% of the patients were satisfied with the results and were able to resume their usual professional and sporting activities. Pinch and grip strength improved when compared with the opposite hand. There were no implant dislocations and the adaptive mobility of the implant was confirmed with dynamic x-rays in flexion, extension, and deviation. The carpal height was maintained in all cases. The radiolunate angle remained unchanged in 15 cases, was improved in six cases, and was worse in four cases. Two poor results were connected with severe ligamentous instability that was present prior to surgery.

**Fig. 35.6** **(A)** Left dominant hand in a 48-year-old patient, light manual worker, sportsman (tennis and sailing.). **(B)** scaphoid nonunion advanced collapse stage III (with dorsal intercalated segment instability of 25 degrees, and capitate erosion). **(C)** Intraoperative view of capitate lesion after proximal scaphoid removed. **(D)** and **(E)** Dynamic x-rays 8 years postoperative. Correction of carpal instability by a palmar capsulodesis, with an associated styloidectomy. Complete return to his professional and sports activities.

▪ Clinical Results

We performed a retrospective study of 20 patients (18 men and two women; mean age, 43 years) who received an APSI implant for advanced degenerative arthritis secondary to chronic scaphoid nonunion since 1989. There were six cases of SNAC stage I, 10 cases of SNAC stage II, and four cases of SNAC stage III (SNAC stage I: radioscaphoid osteoarthritis; stage II: radioscaphoid and scaphocapitate osteoarthritis; stage III: radioscaphoid, scaphocapitate, and capitolunate osteoarthritis) ( Table 35.2 ).

All patients already evaluated in 2000 were tested again in 2007 by an independent observer using the EVAL database system.15 The mean follow-up was 12 years (range, 9 to 14 years, Table 35.3 ). The dominant side was involved in 65% of the patients ( Table 35.2 ). There were 15 cases of SLAC and scaphoid chondrocalcinosis advanced collapse (SCAC), which were excluded from this study. Six nonunions were judged to be type I according to the Schernberg classification15 ( Table 35.4 ) ( Fig. 35.7 ), two of which were failures following volar fixation. Eight cases were type II and six cases where type III. In all cases, pain was the predominant indication for surgical treatment.

**Table 35.2** **Preoperative Series**
			Indications before Surgery
Patients	Age	Etiology	Site	Work	DISI (degrees)	Styloidectomy Associated
1	53	SNAC I	ND	HW	15	+
2	33	SNAC I	ND	L	0	+
3	46	SNAC II	D	L	0	+
4	39	SNAC II	ND	HW	25	+
5	49	SNAC II	D	L	0
6	45	SNAC II	D	HW	12	+
7	32	SNAC I	ND	L	0	+
8	43	SNAC I	ND	L	0
9	38	SNAC II	D	HW	15	+, Ligto
10	49	SNAC III	D	HW	20	+, Ligto
11	46	SNAC III	ND	L	0	+
12	28	Staple 2 years <	D	HW	0
13	70	SNAC II	D	L	10
14	47	SNAC III plus styloid <	ND	L	0	+ <
15	45F	SNAC II	D	L	0	+
16	42F	SNAC I	G/D	L	15	+, Ligto
17	44	SNAC I	D	L	0	Ligto
18	45	SNAC II plus Vis <	D	HW	20	+, Ligto
19	55	SNAC II	D	L	25	+, Ligto
20	39	SNAC II	D	L	10	+, Ligto
Abbreviations: DISI, dorsal intercalated segment instability; HW, heavy work; L, light work; D, dominant side; ND, nondominant; Ligto, ligament reconstruction; SNAC, scaphoid nonunion advanced collapse.

**Table 35.3** **Clinical and Radiographic Results at Follow-Up**
			Results
Patients	Follow-Up (years)	Pain	Flex/Ext [degrees (%)]	Grip (percent)	Pinch (percent)	DISI (degrees)
1	14	1	105 (87)	71	100	10
2	14	3	116 (96)	75	95	0
3	14	1	110 (91)	118	125	0
4	12	5	70 (55)	75	180	15
5	13	3	114 (98)	76	95	0
6	12	2	85 (70)	80	170	10
7	14	1	110 (95)	100	110	0
8	10	2	100 (95)	90	95	0
9	10	3	100 (89)	90	100	10
10	9	2	110 (95)	90	100	0
11	10	2	100 (90)	90	110	0
12	9	1	120 (100)	110	120	0
13	14 + SWA	1	105 (90)	86	80	10
14	10	2	100 (95)	80	90	30
15	13	1	110 (95)	100	90	0
16	9	1	110 (100)	90	100	5
17	9	1	110 (105)	95	100	0
18	11	2	105 (95)	95	100	0
19	9	1	95 (90)	95	90	10
20	10	1	100 (90)	95	100	0
Abbreviations: DISI, dorsal intercalated segment instability; Swa, Swanson trapezometacarpal implant. Swanson trapezometacarpal implant.

Subjectively, 100% of the patients were very satisfied with the surgery. Eighteen patients resumed their professional or sports activities at the same level as before. Only two patients who were manual laborers with work-related claims changed job positions. Pain, which was present in all cases preoperatively, resolved in 15 cases (75%). In five patients (25%), it remained intermittent and consisted of minimal discomfort in daily activities. The flexion-extension arc improved with a mean 100 degree ulnar deviation, which was often normal preoperatively, remaining unchanged. Radial deviation improved in 80% of the cases, and was unchanged or diminished in four cases (20%), three of whom underwent a styloidectomy.

Grip force, assessed with an electronic Jamar dynamometer, was reduced by 10 kg as compared with the opposite side.

Key pinch was pain free and within normal limits as compared with the opposite in all but three patients (one patient had a Swanson trapeziometacarpal implant). There were two failures in our experience: one with a SLAC stage III 7 years ago and one with a SNAC stage II wrist with major instability 3 years ago.

Radiological Results

There were no implant dislocations radiographically in this series. In all the cases, carpal height was maintained, with a mean Youm and McMurtry index of 0.49. The scapholunate and capitolunate angle measurements were within normal limits in most of the patients, with improvement of the DISI deformity in five cases and worsening in one case (this patient did not have associated ligament reconstruction). These values were maintained over the long term, confirming the efficacy of the implant.

**Fig. 35.7** Schernberg classification.

Similarly, dynamic x-ray studies confirmed the implant’s adaptive mobility with no scapholunate diastasis or any other sign of intracarpal instability. A radial styloidectomy did not compromise implant stability in any case.

There was little visible evidence of bony remodeling and the implant remained congruent with the radial styloid. If a styloidectomy is necessary it should be limited to 4 mm to reduce the risk of ulnar translocation, as recommended by Nakamura et al.16 Remodeling the head of the capitate was inconstant (50% of cases), appeared relatively early (2 years postoperative), and did not worsen over time. It seems to be related to any preexisting osteochondral lesions that were present preoperatively rather than postoperatively ( Fig. 35.6 ).

**Table 35.4** **Schernberg Classification of Scaphoid Fractures**
Type I	Fracture of the proximal pole
Type II	Fracture of the upper third
Type III	Fracture of the middle body
Type IV	Transverse fracture of the waist
Type V	Fracture of the lower third
Type VI	Fracture of the tubercle
Source: Schernberg F, Elzein F, Gérard Y. Anatomo-radiological study of fractures of the carpal scaphoid bone: problems of abnormal callus. Rev Chir Orthop Reparatrice Appar Mot 1984;70(Suppl 2):55–63.