The first metatarsal is the shortest and strongest of the five metatarsal. It has two articular surfaces. Proximally it articulates with the medial cuneiform and distally with the base of the first proximal phalanx. It is best described using three anatomic segments, the base, the shaft, and the head. There are numerous ligamentous and tendinous attachments. The base is roughly triangular with an inferior, lateral, and medial boarder. The articular surface of the base is reniform with the hilum facing laterally and a transversely oriented concavity. The tibialis anterior tendon inserts at a tubercle present to the medial-inferior boarder junction. The peroneus longus inserts at a tuberosity present at the junction of the inferior and lateral surfaces. The dorsal and plantar cuneometatarsal ligaments attach to the medial and inferior surfaces, respectively. The lateral surface of the base has an inconsistent articulation with the second metatarsal [29, 33, 37]. The shaft of the first metatarsal has three surfaces: dorsal-medial, lateral, and inferior. The first dorsal interossei inserts into the lateral surface. The plantar surface is concave in a longitudinal direction and its concavity exaggerated by the inferior plantar tubercle. There are three boarders present, the superolateral, inferolateral, and the inferomedial.

The head of the first metatarsal is wider than it is tall, unlike the lesser metatarsals whose vertical diameter is greater than their transverse diameter. The distal surface is covered in cartilage that articulates with the first proximal phalanx. This distal surface is contiguous with the inferior surface that articulates with the sesamoid bones of the first metatarsal phalangeal joint. There are two facets on this surface separated by a ridge or crest called the media crista (Fig. 2.2).

The proximal phalanx has two articular surfaces. Proximally it articulates with the first metatarsal head and distally with the distal phalanx. The base is oriented transversely with an oval posterior facet that is smaller than the metatarsal head it articulates with. This surface is called the glenoid cavity [31]. The dorsal surface provides attachment for the first metatarsal phalangeal joint capsule and the flexor hallucis brevis tendon at a ridge just distal to the proximal articular surface. The plantar surface provides attachments for the abductor hallucis and the adductor hallucis as well as the flexor hallucis brevis and the plantar plate. The shaft is flat plantar with a small groove for the flexor hallucis longus. The dorsal surface is convex. The head is flat with a trochlear articular surface extending more plantar than dorsal. It articulates with the first distal phalanx (Fig. 2.3).

The distal phalanx has a transversely oriented base. The dorsal transverse tubercle just distal to the articular surface serves for attachment of the joint capsule as well as the extensor hallucis longus. The plantar surface has an obliquely oriented ridge from the base to the distal tuberosity providing attachment for the flexor hallucis longus tendon. The distal phalanx deviates laterally approximately 15° from the proximal phalanx [38] (Fig. 2.4).

The non-articular surface is convex in both bones. These surfaces provide multiple attachments including medial and lateral attachments for the flexor hallucis brevis and medial and lateral suspensory metatarsosesamoid ligaments. Laterally there is attachment for the transvers and oblique portions of the adductor hallucis and the deep transverse intermetatarsal ligament. Medially there is attachment for the abductor hallucis tendon. The sesamoids are embedded into the thick plantar plate and within the flexor hallucis brevis tendon. There are two surfaces, articular and non-articular. The shape and size of these are variable [31], though the medial or tibial sesamoid is consistently larger than the lateral or fibular sesamoid. The articular surface interfaces with the inferior portion of the first metatarsal head. The sesamoids are concave longitudinally and slightly convex transversely. The sesamoids are primarily connected to each other via the plantar plate, but there is a thin fibrous band also noted termed the intersesamoidal ligament. They have intracapsular connections to the base of the proximal phalanx at the plantar tubercles and attachment to the metatarsal head via the metatarsosesamoidal ligaments. The sesamoids normally move with the phalanx relative to the first metatarsal head.

The first tarsometatarsal joint has been identified as the apex or center of rotational angulation (CORA) of a bunion [20, 25, 27, 35, 39] with the shape of the distal aspect of the cuneiform described as one of the predisposing features in the development of the deformity. Some have argued that the oblique shape of the cuneiform in bunion-affected feet is an inherited atavistic or ancestral trait. A similar obliquity is noted in human fetal development that decreases as the fetus progresses but is retained in other primates. This ancestral trait remains expressed in individuals with bunions. Others argue that the biomechanical flaws cause stress and strain and the obliquity observed is a result of the Wolf and Davis law as the bone remodels in response [1, 8, 24]. One investigator found that the appearance of an atavistic cuneiform was a function of radiographic projection rather than actual intrinsic deformity and x-ray tube angle, foot position, and metatarsal declination angle affected the relative appearance of atavism [40]. They concluded that radiographic measurement of obliquity did not indicate true anatomic structure and that one should look to a source besides cuneiform shape in understanding bunion development. This finding is corroborated by Dayton et al. [4] in a study on the effect of a first metatarsal phalangeal joint fusion on cuneiform obliquity. They found that the one-to-two intermetatarsal angle decreased with the fusion as did the measureable cuneiform obliquity on standard anterior posterior radiograph. Not only did they both decrease, but they did so with a linear relationship. They suggested the metatarsal and cuneiform moved together in multiple planes as the deformity was reduced to change the perspective of the cuneiform, thereby altering what is observed on radiograph. The problem two-dimensional imaging poses to a three-dimensional deformity is a recurring conversation in the discussion of bunion evaluation and treatment.

The findings discussed above suggest the first ray, defined as both the first metatarsal and medial cuneiform, is moving as a unit; that motion or position applied to the first metatarsal is translated to movement of the cuneiform and in a linear fashion. For this to happen there would need to be very little motion available at the first metatarsal cuneiform joint. Just how much motion takes place at the first metatarsal cuneiform joint (TMTJ1) is debatable, and while there have been multiple studies that attempt to answer this question, many questions remain. First, there is poor reproducibility and validity with subjective evaluations. Second, measurements of mobility with assistive devices are unable to effectively isolate the metatarsocuneiform joint from the first ray as a whole. An extensive review of the literature on first ray mobility was performed by Roukis in 2003 highlighting an additional problem when finding and answer to how much motion takes place at the first tarsometatarsal joint: the fact that no clear consensus exists regarding direction and range of motion [30]. Additional inquiries into the question of hypermobility have been performed since Roukis’ review. One such study, performed by Martin et al. [23], used dynamic fluoroscopic assessment of the foot through gait with full weight bearing. They observed 14 healthy feet and compared these to 8 ft that demonstrated clinical hypermobility and were scheduled for surgical correction of their bunion. The investigators found that maximum dorsal displacement of the first ray was 13.63 mm and 13.06 mm in the normal and bunion-affected patients, respectively, with a mean of 5.27° and 5.56°in the same groups. These values did not show statistical difference in the first ray motion. They also looked at relative translations of the osseous segments and found an average of only 2.61° of sagittal motion at the first metatarsal cuneiform articulation. An average of 5.63 and 4.83° of sagittal motion were observed at the cuneonavicular (CN) articulation and the talonavicular (TN) articulation , respectively. Maximum sagittal plane motion was found at the CN and TN articulations with comparatively little TMT1 motion observed.

Proximal motion may be the reason that persistent instability in multiple planes is retained at the first ray following first tarsal metatarsal joint arthrodesis. Galli et al. [11] performed a cadaveric study in which sagittal plane motion of the first ray was assessed before and after TMTJ1 joint fixation . They found the sagittal motion of the first ray was 7.45 mm prior to fixation and 4.41 mm following fixation. It was only after addition of intermediate cuneiform fixation from the base of the first metatarsal that they found significantly enhanced sagittal plane stability of the first ray. Fleming et al. noted intraoperative transverse plane instability of the first ray as evidenced by their hook test following TMTJ1 fusion . They showed transverse deviation of the first metatarsal with widening of the one-to-two IMA as they transversely stressed the fixated first ray and hypothesized that intercuneiform instability was the cause of retained instability. They proposed routing “spot welding” of the bases of the first two metatarsals to combat this instability [10]. Feilmeier et al. performed a cadaveric study to assess instability following TMTJ1 fusion [42]. After fixating the TMTJ1, they placed screws from the first ray into lateral osseous structures with in varying configurations and measured changes to the common hallux valgus measurements with transverse and frontal plane forces applied. Fixation of the TMTJ1 did not stabilize the first ray in the transverse or frontal plane. They also found that neither a screw from the medial to the intermediate cuneiform nor a screw from the base of the first metatarsal to the intermediate cuneiform stabilized the transverse or frontal plane to a significant degree. Only a screw from the base of the first to the base of the second metatarsal was able to significantly diminish multiplanar motion of the first ray. In all of these studies, it is clear that instability in multiple planes continues following TMTJ1 fusion indicating that motion of the first ray is not primarily at the TMTJ but comes from other intertarsal joints.

Geng et al. [12] performed an in vivo 3D CT study to assess the first ray hypermobility. Ten control and ten bunion-affected patients with a total of 20 ft in each group were observed. They found that during weight-bearing conditions of the foot, the first ray was pronated or everted from its non-weight-bearing position in all patients with the medial cuneiform more pronated than the first metatarsal. The degree of pronation was significantly larger in the bunion-affected feet. The TMTJ1 did show increased motion in bunion-affected feet in both the sagittal and frontal planes with 1.2° more sagittal motion and 1.19° more of frontal plane motion than the control feet. And, while the TMTJ1 joint did invert when compared to the medial cuneiform, the whole first ray was pronated. The findings are consistent with multiple other investigations that very little motion is present at TMTJ1 and that instability of the first ray in multiple planes exists at a proximal level. Their findings also confirm multiple observations of an everted or pronated first ray in a bunion-deformed foot compared with non-affected feet.

Despite findings such as Xiang’s regarding pronation of the first ray in bunion-affected feet, evaluation of normal vs abnormal position of the first ray and first metatarsal has traditionally focused on the transverse plane aspect of the deformity. In 1951 Hardy and Clapham attempted to describe normal and abnormal positions of the various osseous segments involved in bunion-affected feet. The first metatarsal, hallux, and tibial sesamoid position were included in the assessment. They took weight-bearing anteroposterior (AP) radiographs of 252 control feet and 177 affected feet and performed angular evaluations of the various joint segments. They concluded that the transverse plane angular position of the first metatarsal relative to the second metatarsal in a normal foot averaged 8.5° and 13.0° in affected feet [14]. This deviation of the first metatarsal toward the midline of the body is a universally acknowledge component of a bunion, and the angle’s severity is often used to define procedure selection. The position of the first metatarsal in a bunion is reflected in the term metatarsus primus va rus coined by Truslow in 1925. The term as used by Truslow refers to the angulation of the first metatarsal toward the midline of the body in the transverse plane. He felt this term was more reflective of the deformity and intended to move the mind away from the lateral deviated hallux toward what he felt was the primary level of the deformity, the medially deviated first metatarsal.