Ribcage Biomechanics

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The thoracic cage, a complex structure composed of the thoracic vertebrae, twelve pairs of ribs, the sternum, and interconnecting costal cartilages, serves dual critical functions: protecting vital intrathoracic organs and facilitating the mechanical process of ventilation. Its bony framework provides essential attachment points for numerous muscles involved in respiration and trunk movement. Furthermore, the integrity of the rib cage significantly contributes to the overall stability of the thoracic spine, particularly against rotational and bending forces.

Individual Ribcage Components

See also: Ribs

Anterior view of the ribcage. The true ribs are 1-7, false ribs 8-12

Rib Types: Ribs are classified based on their anterior attachments. True ribs (1-7) articulate directly with the sternum via individual costal cartilages. False ribs (8-10) have costal cartilages that connect to the cartilage of the rib superior to them, forming the costal margin. Floating ribs (11-12) lack any anterior attachment and terminate within the posterior abdominal musculature.

Costovertebral Joints (Posterior): These synovial joints connect the head of each rib to the vertebral bodies. Typically (ribs 2-9), the rib head articulates with facets on two adjacent vertebral bodies and the intervening intervertebral disc. Atypical ribs (1, 10-12) articulate with only their corresponding vertebral body. These joints permit the slight gliding and rotational movements necessary for rib elevation during inspiration.

Costotransverse Joints (Posterior): These synovial joints connect the tubercle of ribs 1-10 with a facet on the transverse process of the corresponding thoracic vertebra. The shape and orientation of these joints vary along the thoracic spine, influencing the type of motion facilitated. In the upper thorax, the joints are more conducive to rotation around an AP axis (pump-handle), while in the lower thorax, they are flatter, allowing more gliding (bucket-handle). The axis of rotation for each rib effectively passes through the costovertebral and costotransverse articulations. These posterior articulations are crucial for thoracic spine stability, resisting excessive rotation and lateral bending.

Costochondral/Sternocostal Joints (Anterior): The costal cartilages connect the anterior ends of ribs 1-10 to the sternum (directly for ribs 1-7 via sternocostal joints, indirectly for ribs 8-10). These cartilages provide necessary flexibility and contribute significantly to the elastic recoil of the chest wall during passive expiration. The integrity of the costosternal connection is a primary determinant of overall thoracic spine stability and kinematics, particularly sagittal stability.

Range of Motion (ROM): Detailed kinematic studies using 3D motion analysis systems reveal that during quiet breathing in a seated posture, points on the rib cage typically displace cranially, ventrally, and laterally, with average displacements of 3-5 mm cranially/ventrally and 1-2 mm laterally for the upper rib cage. Quantitative measurements of angular ROM vary depending on the methodology and rib level studied. For instance, studies analyzing CT scans from FRC to TLC have reported average pump-handle movements of approximately 10-16° and bucket-handle movements of 8-14° for the upper ribs. The costovertebral joints inherently allow only limited gliding and rotation, but these small posterior movements translate into larger excursions at the anterior rib ends.

Ribcage Biomechanics with Respiration

The fundamental principle of respiration involves changing the volume of the thoracic cavity to create pressure gradients that drive airflow. Expansion of the rib cage during inspiration increases intrathoracic volume, leading to a decrease in intrapleural and intrapulmonary pressure relative to atmospheric pressure, thus drawing air into the lungs. This expansion is a synchronized activity primarily driven by the contraction of the diaphragm and the intercostal muscles, although accessory muscles contribute during forceful breathing or respiratory distress.

Table 1: Summary of Normal Rib Movements during Inspiration

Rib Group	Primary Motion Type	Primary Plane	Key Joints Involved	Primary Muscles Involved	Effect on Thoracic Diameter
Upper (1-6/7)	Pump-Handle	Sagittal	CV, CT, SC	External Intercostals (esp. Lateral), Scalenes	↑ Anteroposterior (AP)
Lower (7-10)	Bucket-Handle	Frontal	CV, CT, CC	External Intercostals (esp. Dorsal), Diaphragm	↑ Transverse
Floating (11-12)	Caliper/Pincer	Variable	CV (no CT/Ant.)	Diaphragm, Posterior Abdominals	Minimal, Stabilize

The primary movements characterizing rib cage expansion are (see table 1):

Pump-Handle Motion: This movement predominantly involves the upper ribs (typically defined as ribs 1-6 or 1-7). During inspiration, the anterior ends of these ribs and the sternum elevate, moving superiorly and ventrally. This action increases the anteroposterior (AP) diameter of the thorax. This motion occurs primarily within the sagittal plane. Finite element (FE) modeling studies suggest that the external intercostal muscles located in the lateral region of the rib cage exert a significant influence on driving this pump-handle movement.

Bucket-Handle Motion: This movement is characteristic of the lower true and false ribs (typically ribs 7-10). Contraction of inspiratory muscles causes the lateral portions of these ribs to elevate and swing outward, analogous to lifting a bucket handle. This action primarily increases the transverse (lateral) diameter of the thorax. Bucket-handle motion occurs predominantly in the frontal (coronal) plane. FE models indicate that the external intercostal muscles situated in the dorsal (posterior) region have a greater impact on facilitating this bucket-handle movement compared to pump-handle motion.

Vertical Diameter Changes: The superior-inferior dimension of the thoracic cavity is primarily altered by the contraction and relaxation of the diaphragm. During inspiration, the diaphragm contracts and flattens, descending towards the abdomen and increasing the vertical thoracic volume. During passive expiration, the diaphragm relaxes and recoils upward.

Relative Contribution: The relative prominence of pump-handle versus bucket-handle motion varies with the depth of breathing. Studies using CT imaging have shown that during large volume changes, such as moving from functional residual capacity (FRC) to total lung capacity (TLC), pump-handle movements are substantially larger (e.g., four times greater) than bucket-handle movements in healthy individuals. However, during normal tidal breathing (smaller volume changes), the magnitude of bucket-handle movement becomes relatively larger, potentially exceeding pump-handle movement.

Ribcage Biomechanics on Thoracic Spine Stability

Brasiliense et al. demonstrated in eight human upper‑thoracic cadaver segments that the rib cage provides the majority of mechanical restraint to the thoracic spine, accounting for roughly 78 % of overall stability (see table 2). The sternum, true ribs and costovertebral articulations act as an integrated “fourth column.” (See Three Columns of the Spine). Sequential resections revealed the following:.

• The sternum and anterior ribs function as a tension band that chiefly limits flexion and extension—sternectomy alone increased sagittal range of motion (ROM) by 74 % in flexion and 260 % in extension—
• The posterior rib cage, particularly the costovertebral joints, is the principal check on lateral bending, with bending ROM remaining largely unchanged until ≥ 75 % of the posterior ribs were removed.
• Axial‑rotation stability declined in near‑linear proportion to the amount of rib removed, culminating in a 9‑fold increase in rotation after complete rib disarticulation.

Despite these large changes in ROM, coupling between lateral bending and axial rotation remained mild and unchanged, while the sagittal plane axis of rotation shifted unpredictably when the cage was intact but settled near the posterior disc space once ribs were resected. Collectively, the findings underscore the rib cage’s critical, region‑specific contributions to resisting thoracic motions and support its conceptualisation as a structural column that must be considered in trauma assessment and surgical planning.

Table 2. Biomechanical contribution of rib‑cage sub‑structures to thoracic‑spine stability

Rib‑cage region	Flexion / Extension	Lateral bending	Axial rotation	Key biomechanical notes
Sternum ＋ costosternal joints (anterior “tension band”)	• Principal restraint – removal ↑ flexion ROM ≈ +74 % and extension ROM ≈ +260 % (stage 2 vs intact). • Midline sternotomy alone produced little change.	Minimal effect until ≥ 75 % of ribs resected.	Modest restraint – sternectomy ↑ axial‑rotation ROM ≈ +190 %.	Acts as an anterior tension band; failure here is the first large jump in sagittal laxity.
Anterior true‑rib arcs (costochondral shafts)	Shares load with sternum; their partial resection (50 % of rib length) added a further +39 % flexion and +103 % extension ROM.	Still limited effect; bending ROM did not rise sharply until ≥ 75 % of rib removed.	Important; each proportional rib loss produced near‑linear ↑ in axial‑rotation ROM.	Overall anterior rib cage (sternum ＋ shafts) provides most sagittal‑plane stability.
Posterior rib cage (costovertebral & costotransverse joints, posterior rib segments)	Secondary role; extension ROM ↑ only +1 % after final disarticulation.	Major restraint – removing the costovertebral joints (stage 5) added +44 % bending ROM, accounting for ~½ of total bending instability.	Significant restraint; disarticulation added a further +16 % axial‑rotation ROM.	Posterior elements are the dominant check on frontal‑plane motion and a key contributor to transverse‑plane stability.
Intercostal muscles ＋ ligaments (left intact throughout)	Passive contributors to all planes; difficult to isolate quantitatively but included in “rib” effect (≈ 78 % of global stability).	Same as flex/ext.	Same as flex/ext.	Provide continuous linkage between ribs, augmenting cage stiffness.
Intact rib cage (whole structure)	Limits overall thoracic ROM – complete removal produced net ↑ ROM of +181 % (flexion) / +702 % (extension).	Bending ROM ↑ only after large posterior resections, total ↑ ≈ +182 %.	Overall cage accounted for **≈ 78 %** of axial‑rotation stability; ROM ↑ +948 % after full rib removal.	Intact cage shifts sagittal axis of rotation variably; coupled axial‑rotation during bending remains mild and unchanged.

Muscular Contributions

A coordinated effort of various muscle groups drives respiratory movements:

Primary Inspiratory Muscles: The diaphragm is the principal muscle of inspiration, responsible for ~70% of tidal volume in the upright position and up to 90% in the supine position. The external intercostal muscles run inferomedially between ribs and elevate the rib cage during contraction. The scalene muscles in the neck assist by elevating the first and second ribs.

Primary Expiratory Muscles (Quiet Breathing): Normal quiet expiration is primarily a passive process, driven by the elastic recoil of the expanded lungs and chest wall structures (including costal cartilages) returning to their resting state.

Accessory Inspiratory Muscles (Forced Breathing): During increased ventilatory demand (e.g., exercise, respiratory distress), additional muscles are recruited to further expand the thorax. These include the sternocleidomastoids (elevate sternum), pectoralis minor (elevate ribs 3-5), serratus anterior, latissimus dorsi, serratus posterior superior (elevate ribs 2-5), and levatores costarum.

Accessory Expiratory Muscles (Forced Breathing): Forceful expiration requires active muscle contraction. The internal intercostal muscles (run inferoposteriorly, perpendicular to externals) depress the ribs. Abdominal muscles (rectus abdominis, obliques, transversus abdominis) contract to increase intra-abdominal pressure, pushing the diaphragm upward and depressing the lower ribs. Other contributors include the transversus thoracis and serratus posterior inferior (depress ribs 8-12).

The distinct pump-handle and bucket-handle mechanisms arise fundamentally from this complex interplay between the specific orientations of bony articulations, particularly the differing shapes and inclinations of costotransverse joints along the thoracic spine, and the regionally specialized actions of muscles like the intercostal groups. This inherent regional specialization in normal function strongly suggests that injuries affecting specific locations (e.g., upper anterior ribs crucial for pump-handle vs. lower lateral/posterior ribs crucial for bucket-handle and stability) will likely have predictable, yet different, impacts on the dominant mode of chest expansion and overall thoracic mechanics following healing.

Resources

ribcage biomechanics to thoracic stability - brasiliense 2011.pdf - 1.05 MB (f)